Hi Alex,
It's not that difficult. Boot the system with an XP or Linux CD (or other OS
media of choice) and run setup. Delete the existing system partition and
create a new one from the free space, format and proceed with the
installation. Be aware that this process removes all data currently on that
volume/partition, so backup anything you want to keep first.
Before you ask, no, there is no downgrading or uninstalling back to XP or
other OS. Removal requires the above steps (the deletion and recreation of
the system volume is important due to subtle changes in NTFS used by Vista
that cause problems if you try to install XP to it).
--
Best of Luck,
Rick Rogers, aka "Nutcase" - Microsoft MVP
Windows help -
www.rickrogers.org