Lost Disk Space after Automatic System Backup in VISTA

Recently I bought an IBM Think Pad R60 IntelCore 2 Duo 2GHz T7200 which came
with VISTA Home Premium.

For the last few weeks I was getting a pop-up each time I switch on my
notebook which said (as I recall) " Schedule System Backup was not
performed as the Computer was turned off, Do you want to backup?" (I cannot
exactly remember the message). So what I did was I clicked "yes" and left the
notebook on for a few hours while I was away.

After I came back I realised that the remaining Hard Disk Space had been
reduced from 35 GB to around 16 GB. I was surprised to see this. Has VISTA
save full copy of my Hard Disk within my C: drive?

Can anyone please let me know what has happened and how to get back the lost
space. I do not mind deleting any backup that has been save somewhere in the
Hard Disk.
Please help.

Regards,

Pujitha