Trouble:
The free disk space reduces automatically on computers in case of most of windows XP and Windows Vista computers. This is quite a common problem and there are two simple reasons behind it.

Lets see the problem and the fix in detail.

Fix:
The two main reasons for reduction in free disk space are:

1. Indexing of NTFS Drives for Windows Search
2. System Restore Data Accumulated over time

What is drive indexing and how to stop it?
Indexing is a way of storing the important keywords of the files locations and content in a index space. This way, when you search some file or folder, windows will look in the index of the drive first of all and will not look into the real files and folders at that moment. This makes the searching fast. The Indexing data created for the NTFS drive for Windows search consumes noticeable disk space. You can stop the indexing service, this will free up the space consumed by the index but it will make searching a bit slow.

To stop Windows Indexing for a drive, right click on the drive and in general properties of the drive, un-check the options "Index this drive for faster searching" and click OK.

Now we are half done, lets see the rest half, its also very easy, read on.

How to remove old system restore data?
From time to time, especially when you install new softwares etc on your computer, Windows will create System Restore Points, so that your computer can be restored to a previous state in case some problem comes. You should delete very old restore points which you are very unlikely to use. See step number 3 mentioned in this post to delete the same.