I've been using a program called Glary Utilities Pro do fix up the Windows XP registry (source of many "resource" problems) and the original version is available for free download here:
http://www.glaryutilities.com/gu.htmlYou can also do very well by selectively deactivating things in the startup configuration by running MSCONFIG in the run box and editing (ticking off) entries in the START section! If you find you have deactivated something you need you can check it again and restart to get it back. Anything that ends up as an icon on the right side of the task bar is something that was added to XP by some program you have installed and it all takes up memory and makes the computer run more slowly.
Also download Spybot and use it to clean up all the little mistakes that have been added to your system!
I don't use VISTA so don't ask me anything about it!