Discrepancy with free hard drive space

I have a MacBook Pro about 2 years old now that has a 500gb hard drive. Pretty much since I acquired it (8 months ago) it has said that it only has around 20gb of space - give or take depending what I've put onto it or transferred to externals. However I can't find what's eating up this space.
After reading a few threads on here I downloaded Disk Inventory X and ran into this issue:

As you can see one portion of the program is going along with the 500gb hard drive and the other portion is telling me that it's 90.9gb, regardless it's showing that I have not filled the drive, so what should I do? Am I operating the Disk Inventory wrong?

Get OmniDiskSweeper and check the drive space with that.

Several things:

It appears you have two partitions on the drive. Disk Inventory is showing a portion of the drive as MS DOS = ~15 GB, the other as your MacIntosh HD = 465 GB. Is that correct?

If it is, something is messed up with the partition table because it's not reporting the correct space back to the system. That can usually be corrected by resetting the PRAM. Try doing that and then run Disk Inventory X again.

One more thing... Get rid of Kaspersky AV, use its uninstaller to completely remove it. It's not only not needed, it's known to cause problems with resources and slow the machine down. If you must run an AV program for peace of mind, download and install the free ClamXav 2.2.2

Let us know.

Thanks for telling me about Kaspersky, the previous owner was one of those "download/install ALL THE THINGS" types and I've been trying to filter out all the crap.

The drive is not a partitioned drive, the MS DOS drive is my SD card.

Nothing seems to have changed since removing Kaspersky.

If I'm reading DIX properly, the window title says that you have used 90.9 GB of space. I say that because the title bar value is the same as that for my volume (there's a 600MB difference but I'm guessing that might be rounding).

Can you do the following for us? Open up Terminal and enter the following: What does that give you?