unique

Something went wrong on my computer and I don't know
weather it was a virus or disk corruption, or a registry
modification that caused it.
When I try to open anything using the 'open with' command
the notepad just isn't there. I try to search for
programs and point it to C:\Windows\Notepad.exe but it
still won't appear on the list after that. Is there a way
to fix that problem?
I'm running WinXP with SP1 and every possible update.