My sister has my 13 year old PC in London but it's obsolete because a 100MHz Pentium has long since hasn't made the grade for minimum speed (browser bloat/boggy javascript among other factors). I ditched my old Compaq Armada laptop this year as it suffered the same (125 MHz Pentium).

There are kits to stop humming on cooling fans. You can make your own by finding some thin (rubbery) material to stop metal rubbing on metal, or a big flat surface (like the casing side) from vibrating.

My new-since-Easter HP Pavillion actually came with 3 fans: 1 in the PS, 1 on the Core2Duo and 1 on the back of the case. Actually very quiet and don't have to worry about overheating like on fanless iMacs (since I've added a 500 GB Seagate to supplement the OEM 320GB drive) like my partner's old G3 where a 40GB internal was the limit otherwise it'll overheat.

As for the cursor, it may not be a virus or trojan but some other malware or stuff that's sucking too many computational cycles. Might want to use procexp and see what else is running.