The repair install that is mentioned in the other replies will only work if
you have the retail copy of XP. If you have an OEM version of XP, like
Dell, Gateway, Compq, etc, then repair will not likely be an option. If
that is the case, buy a retail copy of XP, which cost about $200 for the
home edition. However, if you happen to own a license (i.e., CDROM with
key) for win 98 or ME or 2000, you could buy the cheaper upgrade for the
home edition. The upgrade version of XP will ask you to insert the CDROM
for the older version, as proof you own it. Note that DOS, win3.1, win95,
and NT doe NOT qualify for an upgrade to XP.