The best you can hope for is that HP provided you with a "restoration" CD or
floppy. This would boot the PC and restore a copy of the original C:
partitin from a hidden image of that partition. But be warned: using such
a restoration will litterally return the PC to the day you bought it,
meaning that all changes made thereafter will be lost, forever. That
includes all personal files and all installed programs.
If this is what you want to do, and you can not find the CD (o floppy),
contact HP support and see whether they can send you one. They may charge a
small fee for this service.
Alternatively, you an buy a full retail version of XP home for a bit under
$200, and install that. (If you have an old 98 or ME CD you can use the
upgrade version of XP hoe, which is under $100. In either case, a clean
install of XP will also erase everthing else on the C: partition.
Are you sure that you need to format? Most file system errors can be fixed
by opening a command prompt and typing CHKDSK C: /R, then rebooting.
If you can not boot into XP to run CHKDSK, it can also be run from the XP
recovery console. This can be run from an XP CDROM (any CDROM with the same
version and SP). It can also be run from a multi-floppy set that is a free
download from Microsoft:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q310994