I have noticed a marked 'slowing down' of my PC recently with the hard drive activity indicator light seemingly permanently lit as if it is constantly processing data or whatever. I had thought it was a rootkit infection (separate thread) but no malware has been picked up by the numerous scans I have carried out. What I have noticed is that the fan in the PC tower sounds very laboured, particularly when I first boot up. I have read somewhere that an over hot machine this can slow things. Looking at the fan it does appear to be somewhat dusty around the air inlet. I have cleared the build-up on the outside but want to know how to clean more thoroughly. Obviously I don't want to go in with a vacuum cleaner on full blast and suck the innards out of the machine! What is the best procedure for this, assuming that this may be a possible source of the problem?

What is the best procedure for this, assuming that this may be a possible source of the problem?

I don't know if this is best, but here's what I do at the shop: I use a non-oil lube air compressor with a 5 micron filter set to about 40 psi with a rubber tipped nozzle to blow the fan out being careful to blow from the side of the fan opposite the exposed bearing and avoiding touching anything with the nozzle. You could also use a can of compressed air (it's actually a chemical) designed for electronics (example: Amazon.com: Memorex 10OZ 152A AIR DUSTER (http://www.amazon.com/Memorex-10OZ-152A-DUSTER-32028021/dp/B0007YZTSC)).

Any fans accessible from the outside of the computer are either general cooling or power supply fans. The fan of interest is the processor fan and heatsink. Modern computers suck in so much air that a dust blanket can form on the heatsink. This development is hastened if the computer sits on the floor. If the computer has some age, the fans could also be worn out and may growl. A clue will be the warranty length. One year warranty models tend to have budget fans.

A constantly running hard drive activity light is usually due to a process running in the background, like an antivirus scan, or insufficient memory for the number of processes running. In the latter case, the drive activity is due to the swap file being in constant use. Windows' task manager (CTRL-ALT-DEL) will show both CPU utilization on the processes tab and peak memory use on the performance tab. If peak commit charge is more than physical memory, the swap file will be in significant use. The more disparate these values become, the more swap space is used and the more the drive will be active. Adding memory gives more performance bang for the buck than any other upgrade other than wholesale replacement of motherboard and processor.

Resources:
Wikipedia: Commit Charge (http://en.wikipedia.org/wiki/Commit_charge)

Thanks Scleros. I know I need more memory but the slow down seemed too sudden and a bit of a coincidence with the fan noise.

I reread your original post. Bootup fan growl that then diminishes as everything comes up to operating temperature is typically due to a worn fan. Most of the time the culprit is the processor fan, for which the computer will need opening up to access.

I reread your original post. Bootup fan growl that then diminishes as everything comes up to operating temperature is typically due to a worn fan. Most of the time the culprit is the processor fan, for which the computer will need opening up to access.

In addition, some cases where the fan is readily accessible and can be removed, remove it and GENTLY remove the sticker face covering the fan bearing and apply one or two drops of wd-40. Reseal and reattach, I've saved several fans this way, very cheap and simple fix. If this doesn't at least help the issue, then I would second the statement that your fan is further gone than what my suggested help can even do for you.