Dell Inspiron Mini won't shut down

I've had this problem on my xp machine. The problem was my antivirus downloading updates which stopped the computer shutting off.
Another problem might be a windows error. You can try and fix this by using the recovery disc if you have one for xp?
There are a lot of problems that this could be unfortunately, a virus could be messing around with your pc, run a full system scan and see if that works.

My eventual fix was to reinstall xp completely and leave it at that...

run error checking tool on your main drive
right click on it> properties > tools

odds are if there isn't anything connect via USB ports when attempting to shut it down then it's software related.