Disk space available error

My computer shows that my 4Gb hard drive is 99% full. I
have deleted internet files, cookies, temp files, some
large programs and emptied the recycle bin. I also ran
Norton Systemworks defrag (system optimization), clean
sweep and disk doctor. And also have run scan disk and
no errors came up. Is this more likely to be a hardware
problem or a software problem. Also is there anything
else I should try to be doing. Thanks.

Thomas Chen