I have a Dell Dimension 5000, Windows XP, Intel Pentium 4, which i got second hand some years ago. Some time ago the PC began to run really slowly, and intermittently the fan becomes very loud, as if the machine wants to take off. This has been gradually getting worse, to the point where i have to reboot regularly. I have virus scanned, with AVG, didn't appear to be any problems.
I decided i would attempt to wipe the disk and re-install XP from scratch. Before that, however, i thought i'd try to give it a clean inside, so opened the case and brushed away a good few years of dust from the motherboard, fan, graphics card, power pack etc. Closed the case, and now the fan starts up extremely loudly, there are six, what i would say sound like warning, buzzes (can't locate exactly where from), and no signal to the monitor.
The disk appears to be spinning, the DVD-ROM drive works, and the USB ports work (checked that by connecting a portable drive - the light goes on). The monitor is working.