Memory almost full but can't identify why

I have a Lenovo Thinkcentre PC, with windows XP. I'm having a problem with the PC in that although I use an external hard drive to make sure the PC hard drive is not full and have very little saved on the actual PC, when I look in properties on the 'C' drive by right clicking on the 'C' drive icon, it is showing that there is 118GB used and only 26.1GB free space (total capacity 144GB). But when I go into the 'C' drive and manually explore all the different folders, there is nowhere near this much space used. Probably less than 5Gb that I can account for. I think it is drastically slowing my system, and also every few days I get a message that my computers performance
is degraded and that I should defragment. I was under the impression that you didn't need to defragment anywhere near this frequently. Any ideas what this could be? Any advice much appreciated! I'm not great with PC's! I even thought of getting more memory for my PC, but that was scuppered when i can't even find out what model PC I have!: When I look in control panel
in the general tab, I get the info: 'Lenovo Intel (R) Core (TM) 2 Duo CPU E4500 @ 2.20GHz, 2.19GHz, 1.99 GB of RAM'. In the computer name tab it gives 'full computer name' as 'LENOVO-D45C47F7'. And somewhere else i got the no's: 'LMBKV51'. Unfortunately none of these are lenovo models which I can find on the lenovo website!

 

Try installing this program to see where all your space is being used: http://windirstat.info/

 

how about hibernation?