freeup hard Disk

I have Pentium II IBM system
with 3 GB Hard Disk with two equal sharing.
In C there is only Window 98 and MS Office but i have only 40 MB spare hard disk in hand.
The reason is that previously i had a number of software installed but deleted later on. And in my views most of space is occupied by the unsed files.
I want to freeup my hard disk from such unused files without formating.
Do you have any suggestion in this regard.