Working in the enterprise market for many years now I have seen a new solution gradually taking market shares.
The solution is virtual computers running inside a big server.

You can have several instances of Windows XP, Windows server, Linux, FreeBSD and family all running happily and acting like a machine inside the main machine.

But you do not need to be in the enterprise market to enjoy these functions.
These products are readily available for you as the end user aswell, and directly on the desktop of your laptop or home computer.

The solutions I would recommend for this is either Parallels, VMware or the free alternative Virtualbox.
Running any one of these systems will give you the possibility of installing another operating system on your already existing computer.

Some of the advantages of this include:
-The possibility of installing software inside the virtual computer which you are uncertain of the stability of, thereby protecting the integrity of your main host operating system.
-Using MacosX as the host operating system and having windows installed inside of MacosX to run software that do not have any MacosX install candidate (Applies only to Intel Macs).
This can ease your transition from Microsoft Windows to MacosX
-Testing of other operating systems, like Windows Vista or Linux without removing already existing systems.

If your main system is Vista you risk some programs not working, but with a virtual computer inside of vista you are able to run applications inside the virtual computer while still running Vista.

The three mentioned before are all good alternatives for the end-user market.

Here are some advantages of each alternative:

Vmware:
The “Unity” feature makes windows applications run seamlessly on your MacosX desktop, making the Microsoft windows applications readily available to move and interact with your MacosX desktop.
Bootcamp integration: Bootcamp is the program from Apple that makes you able to chose between booting MacosX or windows XP, some VMware versions has the abilty to use you’re your bootcamp able operating system alongside MacosX.
VMware is the mature contender in this, not only do they have a broad base of customers amongst end-users, but they are enterprise ready as well, many customers in the company I work in myself do have their mission critical systems running on on VMware. Can run MacosX inside a windows machine, this does require some research and work.

Parallels:
Parallels has a feature which bears the name “coherence”, this feature basically does the same thing as the “unity” feature in Vmware.
Parallels can also boot up filesystems that are available through bootcamp.
Parallels is the younger challenger of the two, their “coherence” feature was a great boon to the user community, this feature forced VMware to create their “unity” (desktop integration) feature.
Parallels can run MacosX inside a windows machine, this does require some research and work.

Virtualbox:
Its free stable, desktop ready and the source is open, open source means any user that has the skill and time needed can improve and help develop the product. It can be downloaded freely and and no one will ask for your credit card number.

In the real world i have ran both VMware and Parallels for some time on many different computers, I started out using VMware on my desktop, but I changed to Parallels because of their support for MacosX.

They in addition kept me using their software by creating great features like the “coherence” mode.
Later VMware has picked up the glove and seemed to have put technical resources aimed at the MacosX market, and I have also tried this, though not for such a long time, but I am quite pleased with what I have seen from that side aswell.

So both VMware and Parallels has created great products which you can use as great tools to expand on your computer investment, this is true whether you use Windows XP, MacosX or Linux.

I have not yet tried Virtualbox, but the reviews look great so I might give it a go, and so should you.
If you can fork up for the paid alternatives the best bang for your buck would be if you run Parallels inside of a Macintosh, and running MacosX as your main or host operating system, then the power of XP, Vista, MacosX and Linux can all be at your fingertips coherently and simultaneously.

If you do run it inside of a standard PC components (Dell, IBM, Acer etc.) it will take some time and research to make MacosX run inside the virtual computer, however you can freely run other Microsoft or Linux operating systems at the same time.
Mind you I do say “freely”, but it has a catch, even installing a Micosoft system inside of a virtual computer does still mean you have to pay a license, also the normal OEM license does not allow you to install the OEM licensed product inside the virtual computer.

For more detailed information I would suggest visiting the websites for these systems:

http://www.parallels.com, http://www.vmware.com. or http://www.virtualbox.org/
Both VMware and Parallels has trial versions, this means you can try their product for a while, then after 30 days or so it will not work anymore, and you have to shuffle around your wallet for the credit card.

There is also one solution from Microsoft called VirtualPC, sadly it is usually to slow for any serious work.