I don't use Vista, but Windows 7 is very similar.
If you hold down the "Windows" key (the one with Start or the windows logo on it) and press "E", then release them both, the Windows Explorer should appear.
You can also get to it via the Start menu.
That will show you the drives on your machine. You'll see something like "Drive C" and 65GB free of 112GB (or whatever).
That will tell you how large your Drive C is. If any other hard drives appear, you can see what they are and how big they are.
If Drive C is a lot less than your nominal hard drive size, and no other drives appear, it probably means you have a recovery partition on the drive. That's not unusual.
If you need more HD space, there are three options:
- replace the existing drive with a larger one (make sure it's the same type - PATA or SATA: they aren't interchangeable in a Thinkpad)
- an external USB drive, or
- a caddy to slot in the drivebay where the CDROM/DVD drive lives.