Two pieces of basic advice :
Try making some room on your HD (defraging requires moving bytes around to be able to create contiguous files). If you have large files, you will need a rather large amount of free space. Try cleaning up the temporary folders and files, and possibly the IE junkyard.
If this not enough, and you have a CD burner, backup some files and remove them from your HD
Follow rickity's advice and run in safe mode (no drivers loaded - incl. video - means more available memory)
HTH