yes you can...if you're lucky it'll possibly work straight away...that is VERY rare...
otherwise, you have to do a "Repair" of XP...which will leave your registry and file completly instact...it is not 100% garrenteed to work...but its your best bet without loosing data...
you have to boot from the Windows XP Cd, go through the process as if you would install it like normal, and at one stage of the setup it'll "Search For Existing Copies of Windows", when it does that it'll list them, and you select your WinXP and press 'R' for repair...it would only come up with one copy of Windows, unless you dual boot...
after you do that it mimics the normal windows installation almost exactly...
and you will need re-activate your copy of WinXP...
and do not do the first repair option it gives you...the first screen it gives you will have Press Enter to install, R for Recovery Console or F3 to quit....the recovery console is useless!!!!
however, this has been known to make PCs BSOD...but it also does that before you try it, so it doesn't make much of a difference...its a good idea to back-up everything anyway...