PhatIce using any of the skins and themes here will not slow done your computer. A lot of folks are still running XP, as I do and have not had any slow down of operations. Having said that, the amount of memory you have on your computer and your processor will effect the speed of your operations. I have a P4 2.6Ghz processor with 2.0GB of ram and run XP Home SP2 and don't have any problems.
Now you do understand that in order to use any of the skins or themes you will need a program to run them for instance if you want to download a WindowBlinds skin you will need the WindowBlinds program to run it. If you are interested in customizing your desktop why don't you check out the following link. It's to the WinCustomize Wiki and should answer alot of questions for you. Then come on back if you need help.
WWW Link