What can I turn off on windows XP to free up memory without breaking everything

I seem to have a lot of services that start up on windows, many of which I have no use of (to my knowledge). are there any services which are on by default which I won't really miss and which will free up my system so it works a bit better.

This question is specific to XP, I have a vista laptop which I will ask the same question for.

Try BlackViper for some good configurations of varying levels for most Windows OSs. For example, XP SP3 is here.

Also, try using the Autoruns tool from Sysinternals to get rid of Adobe and QuickTime startup apps.

Then, uninstall all apps you don't use. Really - do you use that Noughts-and-crosses game all that often?

Finally, use lightweight equivalents of big apps - Foxit Reader instead of Adobe Reader.