Just got my new pc a couple weeks ago with a WD 640gb harddrive. I have been noticing that I am losing gigabytes of space at a time. Just since yesterday morning, somehow I have 2 less gigabytes of available space. I haven't been downloading anything. What could this be?
You don't say which OS you are using. Normally that sort of information is quite useful when answering questions.
Assuming that you are using Vista, or XP, it's probably the daily System Restore checkpoints. Look here for details. Eventually things will reach a steady state, when the checkpoints are using 15% of the disk space, and old checkpoints will be deleted as new ones are created.
If it's a real problem you can always turn the checkpointing off - think carefully before doing so as it will make it far harder to recover from problems.
Windows auto updates, system restore points being created, swap file size changing, Windows writing to log-on/event files everytime you bootup, there are numerous reasons. Nothing to worry about unless it keeps happening and you start noticing big changes, like 20 gig at a time disappearing.
Sorry after I posted I realized I should of stated I'm using Vista. I also looked up the problem and someone else said it has to do with shadow copy in vista. I resized it from 70gb or so to 3gb and gave all that space back to me. Is this ok or do I need to have a lot of shadow copy storage?