AVR-Chat

Hi,
I've been running a few simple LED chaser and PWM programs on my
AT90S8535 and I just went to switch from my old Windows 98 computer to
my Windows XP machine, when I noticed that the RESET pin wasn't working.
No matter what I connect to it, it continues to act as if its connected
to +5V and nothing else - the previously loaded program still runs.  So
now my programmer won't work either.
I've checked the power supply and the continuity between the pin and the
input and they are both fine.
What could be causing this behaviour?
Thanks,
Jack