You need to say what your operating system is and what the size of the HD is.
But some pointers are:
Keeping a lot of e-mails will use up disk space. Mine uses about 750 MB.
If you have crashes regularly, there may be memory dumps in the Windows System directory.
Look in the Temp directories for garbage. There could be two of them; one off the Root directory and one in the Documents and Settings branch. (Depends on OS).
Using Windows Explorer, do a search on either all files, or on files over, say, 50KB. You'll soon find out whether your space is being consumed by garbage or stuff you are saving for a rainy day.
NT4 used to keep a file called something like Handler.trc and it grew and grew until at 1GB I deleted it and suffered no ill effects.