Computer doesn't sound right.

For the record: Acer 3600GT, four years old, running XP Home. New HDD installed four months ago.
This machine isn't exactly soundproof, so I'm accustomed to hearing a certain pitch in the whirring of the machinery. Normally, especially when it's idle, it settles down to a low purr.
Just since yesterday it seems to be in a near-constant state of activity. And even something like opening Firefox to go online sets it running higher.
Now and then - as it's doing right now, for example - it starts labouring briefly like a car starting up a hill. I've checked Task Manager and I can't see any unusual processes running. The CPU temperature is moving between 62 and 66C, which is a little higher than normal, but we have had a sudden heatwave here.
I've run scans with Spybot and NOD32 and nothing nasty showed up.
Anyone got any ideas?

 

Thanks for the quick response.
The temperature has gone down some since my first post. I've opened the window, the current temp readings are 61 for CPU and 31 for system.
I give the fans a light blast of tinned air a couple of times a year - you're probably right, they're about due.