jBear's right, prefetch is the OS's ability to "pre" load frequently accessed programs to make those programs start faster, but that does suck up RAM to do that.
If your havving a problem with low resources, you can turn off pre-fetch, which frees up resources, but makes programs run slower. THe details on how to do it are here:
http://www.tweakxp.com/tweak1796.aspx