I've found that getting a *good* cou cooler to be considerably more
critical
than the case the machine is built in. I've purchased quite a few cpu
coolers
that just plain did not do the job...even though...according to their
ratings
they should have worked.
I think I'd replace your cpu cooler with a better one.

I took a peek at the website for the cpu cooler you have...
and I have a similar unit (not sure if it;s the same one)
I found it worked well with cpu's of 1 ghz or less...but with a fatser cpu
i was getting "higher" cpu temps

I had the original CPU cooler on it then changed to Coolermaster then
changed to the Thermaltake SilentBoost. The SilentBoost is rated up to XP
3400 and has got excellent write ups everywhere I've looked. I'd gladly
change it again if there was a cooler that would do the job but I honestly
think the one that is on it just now is ok. What is concerning me most is
that the temps have been gradually getting worse. Can the Arctic Silver
deteriorate that quickly (8 months).

I'm going to bring the PC downstairs and clean it all and do as Martin has
suggested and turn the fan round first to see if that helps any.

cheers

Steven.