have you been messing with the cables or anything inside the box? Sorry, but I've never heard of a pc that just stopped seeing a HD unless the following occured:
1. bad cable or messed up HD jumpers
2. bad HD
3. boot sector fucked up (virus or accidental)
Win XP dosprompt utility that may help is FIXBOOT ->
http://www.webtree.ca/windowsxp/repa...Boot%20Sector:
and you can read about some others that have had similar problems
http://www.experts-exchange.com/Hard..._21550445.html
you have a boot disk or cd? if not, go here and get one, then try again if that's what you need
http://www.bootdisk.com/bootdisk.htm