Vista will be released soon and many people are thinking about installing it. When XP was launched, some people ended up uninstalling it because a game wouldn't run or some hardware wasn't supported. The safest way to try Vista is to install it in its own partition, keeping XP untouched until you're sure it works.
If you don't have a partition manager, download an ISO image of the GParted Live CD from gparted.sourceforge.net and burn a CD from it. This is a free partition manager based on Linux and an essential addition to any serious PC user's toolkit.
Prepare to resize your XP partition by running the command ‘chkdsk /f /r' from a command prompt – you must restart to allow the check to run. Then run the Windows defragmenter a couple of times to move all the files near the start of the partition.
Back up your files. This is important. Resizing your partition shouldn't destroy any data, but if something goes wrong, it might. Now boot from the GParted CD. Select your NTFS partition and shrink it to leave enough space for Vista. Vista requires a 40GB hard drive with 15GB free, but 25 or 30GB is more realistic.
Boot back into Windows XP to check that everything works, then insert the Vista install CD and boot the PC again to start Vista Setup. Pick the Custom install option, which allows you to choose where you want to install Vista. Tell it to install in the unallocated space you just created. Vista will then be installed, together with a boot manager that will appear when the PC starts, to allow you to choose which version of Windows you wish to use.
When you have finished trying out Vista you can use GParted to delete its partition and restore the XP partition back to its original size again. Alternatively, you can just reformat the Vista test partition and use it as a D drive.