It could be a coincidence. Windows Updates should not affect boot files.

I'm assuming there are no CDs or other bootable media in the system. If so, remove them.

Did you run the hard drive manufacturer's diags? I'd do that first. You can download them from the manufacturer's website or get UBCD5.

After that, you can try a repair install.
http://www.michaelstevenstech.com/XPrepairinstall.htm