Disk space disappearing mysteriously in Windows?

Oh I feel so happy whenever I solve yet another mystery in the wonderful ways that Windows works :-) So happy that I desperately need to blog about it :-P

Last year I bought a ThinkPad laptop with Windows Vista. Lately I was running out of disk space. It just had an 150GB drive, so I was running low. I bought a new drive and with lots of failed attempts and some magic utility finally managed to restore the original disk with its mystery Lenovo partitions into the new disk and have the laptop actually boot! Then I had an extra partition which I decided (I now I am adventurous...) to mount as a folder on my C: drive. Just to see how that will work.

Then I moved lots and lots of stuff to this mounted-partition and got around 40GB free space in the main partition. Plus another 100GB or so on the mounted partition, I thought I was good.

However... the free disk space of the main partition continued to drop and drop and drop without any apparent explanation. When it reached down to 2GB, I moved some other stuff and got it back to 10GB. Then drop and drop again and today I had only 1.5GB free on the main partition. That was a challenge that simply begged to be tackled!