|Might there be too much on the C drive?|
What error's do you get?
The Vista one is good enough. Gparted should work also.
I guess a bad partition table or faulty file system might do that. Try chkdsk and also clean up system. Defrag too.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10