Hard disk space missing where, why and what can i do to fix it?

[silentmarta]

Ok i am using windows vista 32-bit. On my computer it tells my i have 465GB hdd with 371GB left. When i saw this i thought that cant be right becuase theres no way ive used up nearly 100GB in 2 weeks (pc new). If i go into my c drive and select everything the total amount is about 44GB. So whats happened to the other 50GB! if it was only 2 or 3 missing i wouldnt mind but somthings not right. Does anyone know whats happened or what i can do to fix it?
**Update**
Thanks for all your answers guys but i found out by acident what it was. It was all the shadow/system restore files that were taking the space up. Becuase it just got my pc i have been installing programs etc so i assume eveytime you install a program it makes a copy so over the last 2 weeks id made like 100 copies! If anyone else has this problem right click your C drive go to properties then on the general tab go to disk clean up select files from all users in the pop up. Once in disk clean up go to the more options tab then click the shadow and system restore clean up button. Hope this helps!