So, my laptop's HDD had about 200 MB of free space left.
So, I transfered a folder that was more that 700 MBs to the other computer via lan...

After the transfer was completed, I deleted the folder, and I cleared the recycle bin.

When I went to see the free space of the HDD it said that it was around 270 MB of free space!

Where did the 700 MBs I deleted go?

try using FreeRAM XP

I had the same kinda problems a few years ago, solved it by rebooting.

Windows sometimes likes to cache the amount of free space left on the drive - i've noticed it before. After a while it will update the amount of space (or when you reboot as was said).

The other thing to try might be to use the Search for Files and Folders tool to locate files over a specific size, sorted by size and then you have a fair idea at what is taking up so much storage.

Turn on hidden files and folder viewing (if not already on) and locate to C:\Recycler. This is where the recycle bin is actually located. Have a snoop around in there and see if any files seem to be lingering..

and don't forget theres always the Disk Cleanup tool in Accessories which can be helpful

There's a program called TreeSize (google it), it gives you a summary of all your folders with a meter next to them telling you their size.
I used this prog to cleanup my HDD once and I really liked it.