Need help with computer hdd full

I says my hard drive is full but it cant be. I just did a full format and nuked everything. Only things installed are XP (not SP2), Quickbooks and Virus protection. Yet it says my hdd is at full capacity. Its a relatively small drive 18gb, but I sure it is not full. Computer runs like crap now too. Is there a way to test the integrity of a drive? Disk Cleanup wont work either. It just hangs. Think the hdd is toast?