Another vote here against Vista.
Something that may be worth mentioning is that you can get some real bargains if you look for companies selling "factory refurbished" PCs. These are basically PCs that were sold then for one reason or another returned to the manufacturer, who fixes any faults, tests them, and sells them on to a dealer.
Usually they are more reliable than brand new computers because they have been properly tested after assembly (most PCs are only checked during assembly of e.g. the motherboard, then have drives etc. fitted afterwards) and have had all defects fixed. The down side is that you may not get all of the normal warranty protection. The up side is that they're cheaper and may still have XP.
My main workhorse PC started out this way and is four or five years old now; I had to replace the power supply a few months ago because it was failing after I'd put a second DVD drive and hard disk into the PC, but that wasn't difficult, it just bolts and plugs in.
