I have installed SP2 on 4 machines now, all without any problems at all.
1 X Laptop XP Home
3 X Desktop XP Pro
One of the things that Microsoft recommended was that all previous update should be installed before installing SP2, maybe this is where some of the problems have occurred on the different problem machines.
Of course, we could all change to Linux, but now only do you need to know how to trade, but you need to be a programmer as well. Anyone tried installing a kernel update, hey even many of the programmes are a nightmare to install on a Linux system.
If that isn't challenging enough, try "Linux From Scratch", you start with a completely blank computer and bit by bit build your own operating system from the various components - a bit like building a kit car.
I think Roberto has the right idea, if it's very broke, just buy a new PC
In all seriousness, if it is so broke you just can't fix it or live with it, back up your data to somewhere safe eg another machine or onto CD roms. Then take it to a PC repair shop and ask them to either fix it or format and re-load it. This will clean everything off and you will have a machine that was just like it was when you bought it new.
You will of course need to re-install all your programmes and transfer your data file back on, but it may be a much less expensive solution to buying a new machine if you don't need or want to do that.
As for RogerM's problem...
It sounds like the one thing has messed up both the position of the desktop and the background image, sorry I can't tell you what it is. I am much better working at a machine than I am trying to figure out a problem from a distance. I would never make it in a support call centre, probably don't have the temperament anyway.