You don't say what you've tried to free up space on your C: drive so try these :
1. Double Click "My Computer", Right click your C: Drive and click "Properties" then "Disc Cleanup"
You will see how much space can be reclaimed by deleting Temp Internet files etc.
2. Check your System Restore settings ( Start / All Programs / Accessories / System Tools / System Restore ) if running Win Xp - these can use up a lot of your HD space. Delete some older Restore points if you think you won't need them - like if you have a couple of recent restore points and everything is working fine.
3. Don't try moving system files or folders that's asking for trouble.
4. UNINSTALL programs and then re-install them, specifying your other HD during the installation. You can't just "Cut & Paste" a Program from one drive to the other as there will be system files all over the place in addition to those in the Program Files folder.
5. After this do a Defrag of your C: Drive to tidy it up. ( Start / All Programs / Accessories / System Tools / Disc Defragmenter ).
All the best,