Very Odd 2011 iMac Hard Drive Issue

I have an iMac that tells me it's hard drive space is totally full even though I've used barely any space (it fluctuates frequently between 450MB to 160MB free?). It's a late 2011 base model w/ a 500GB HD, purchased about a month ago. I have no idea what is causing this, as I have heard Lion does automatic backup w/ Time Machine and puts the large files on the mac, but I have Time Machine disabled on this Mac. I checked and saw it was my user account that took up the space, but the folders inside it (home icon user folder) and nothing amounted to it. I also used disk Utility which told me the Macintosh HD was OK. What could be causing this recent capacity issue as I've only really used about 1/6 of the space on the drive and it's relatively new... help would be greatly appreciated and thank you for answer in advance!

Something is going on. It could be a left over sparse bundles from Time Machine but the amount doesn't sound right.

Let's do this: Download the free Disk Inventory X, run it and see what it tells you about the space which is being used. Post back and let us know.