Mac Disk Space report confusing - HELP!

Hi - I just did a new install of 10.7 on my MBP and there are strange things happening.

So just going with 'Get Info' .. my 500 GB Macintosh HD has 395 GB available (103 GB used)

When I go with Omni DiskSweeper, it tells me there is 205 GB available (and 293 used)!! What the heck???

Just to make it even more confusing.. after performing the actual sweep, OmniDiskSweeper reports the same numbers as the Mac's 'Get Info' command. So it hasn't picked up the extra 190 GB. Is it some invisible file? Can it really be that BIG???

SO just to clarify.. OminiDiskSweeper has 2 windows open: The Drive List, and the Main Sweep Results window. In the Drive List it tells me I have 293 GB used. And in the other window (with a list of all the files are actually on the machine), it tells me I only have 103 GB used.

What on earth is going on???