You don't have to restart the system to use the OS of choice in a VM, Say you have XP and install VM, then you can load any other OS you want without ever leaving Vista, it runs the older OS "inside" of Vista, pretty sweet.

http://www.microsoft.com/windows/products/winfamily/virtualpc/overview.mspx

Another advantage is when running a virtual OS, it is very hard for viruses and other nasties to get outside the VM and infect the host OS.

One thing about running virtual machines is that you need a lot of RAM...depending on what operating systems you are using, otherwise the host and guest operating systems will be slow. With Vista and XP, you will need quite a bit. But for XP and XP or 98, you might not need as much. Another neat thing about VMs is that you can save/backup the virtual hard disk so you don't need to reinstall the OS again when you want to start over...or to make identical copies of the disk for testing purposes.

I'm assuming you want to install XP. Do you have a retail version or Dell's OEM version? If you have Dell's version, it may not install in the VM because the VM is not a Dell computer. I've tried this on mine awhile back and it didn't work because of this.

It also depends on what programs you are trying out. If they are simple programs like messengers, word processors, etc., a VM should work. However, if they are graphic intensive games or video editors, then another partition/dual booting may be the way to go.