You don't say which OS you are using. Normally that sort of information is quite useful when answering questions.
Assuming that you are using Vista, or XP, it's probably the daily System Restore checkpoints. Look here
for details. Eventually things will reach a steady state, when the checkpoints are using 15% of the disk space, and old checkpoints will be deleted as new ones are created.
If it's a real problem you can always turn the checkpointing off - think carefully before doing so as it will make it far harder to recover from problems.