Disk cleaner is a program which rids your disk of unused and unwanted files, however, have you run a defragmenter program to re-organize the files on your disk to make them faster to access? Here are the Microsoft Windows XP instructions for using the built in defragmenter:
http://www.microsoft.com/resources/d....mspx?mfr=true
How big is your hard disk? Perhaps the hard disk is approaching a point where it is too full to quickly find storage areas for your large graphics files. You may want to look into getting a second larger hard disk for the graphics files.