The easiest solution to a quieter case is to use quiet, low RPM fans and to remove unecessary fans. I'd suggest getting the Alpha PAL8045 heatsink and pair it with a Panaflo L1A 80mm fan (that heatsink requires an 80mm fan). Either buy a quiet power supply or replace the PSU fan with a Panaflo. Get a motherboard that doesn't have a northbridge fan or buy a replacement northbridge heatsink like the Zalman
Northbridge Heatsink. Replace the front intake case fan with a quiet Panaflo.
If the hard drive is a source of objectionable noise you have 2 options: buy a new and quieter drive like the Seagate Baracuda IVs with the special fluid bearings or dump their current hard drive into a
SilentDrive enclosure.
If you do all those things above then your parents' PC will probably almost as quiet as you can get without resorting to more drastic mods and sound dampening material (which can't help temps inside the case). I just did all of the above (well I've had a silentdrive for most of a year) and my PC is very quiet.
A good, cheap source for Panaflo L1As is
BGMicro. If you don't want to go the Panaflo route I've heard excellent things about the Enermax adjustable 80mm case fans though I believe they do cost more than the Panaflos. The L1As are under $3 apiece. I got them for $2 apiece but I think they've slightly raised the price.
Good luck!
Gaidin