For an XP processor I'd recomment a Thermalright AX-7 (or is it Thermaltake? Argh...) heatsink and a fan adapter that goes from the AX-7's 80mm to accept a 92mm fan.
Use a low speed "quiet" model 92mm fan, such as the Vantech stealth or Pabst fans.
For case fans, use as big of a fan as you can (i.e., 120mm or 92mm instead of a 80mm or 60mm). Again, use quiet low speed Vantech stealth or Pabst.
Replace the fan in your power supply with Vantech Stealth or Pabst of same size. Don't worry, it will work fine even if the original was temperature controlled.
If your chipset (northbridge) on your motherboard has a fan on it (40mm) then remove the stock chipset heatsink and replace it with one that needs no fan, such as one from Zalman. Those little 40mm buggers can be noisier than you expect, especially when their underdesigned bearings start to wear.
Most graphics cards aren't too noisy, but if you have a GeForce4 or a few others, you might want to have a close listen to make sure. There are replacement coolers for these, but often they're noisier than the stock fans.
The general idea is to, in every situation, use as large of a fan as you can. Then, use the quietest fan of that size you can. Larger fans typically move more air for the same noise, or the same air for less noise than smaller fans do. Therefore, you'll still retain good air flow rates and cooling capability but reduce your noise drastically.
This would be my first line of attack. If that isn't enough, then I'd consider a sound isolation box, moving the computer into a closet (what I did) or something along those lines.
For heatsinks and fans,
www.plycon.com is a good place to start.