E drive

I have C drive which is the main drive,  And E drive which says its the boot drive.   I cant get it to run off of C drive.  The E drive keeps popping up message saying seriously low disk space please run clean up to free up some drive space.   Well E drive has 3.35 gb  and it is saying it has less then 145mb free space.  the C drive has 24.3 gb and only 5.92 used.  But I cant seem to get it to run off of c drive.  any suggestions.   Running windows xp