|Flashing cursor in the upper left corner?|
if so this is a usual sign that you lost your master boot record. without getting complicated you are missing one of the many pieces of booting to windows or any os for that matter.
Usual course of action is to boot the xp cd and run recovery console. Then run the utilities fixmbr and fixboot.
See if you come up.
If not attempt a repair install.
Source of the issue can be many fold with the usual suspects being virus, hardware failure, and system OS corruption.
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