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 ​