Disappearing Hard Drive Memory. please help!

I've been having a problem like so many others with mysteriously disappearing hard drive space. It has been rather full for most of the time I've had it (i have lots of music and pictures) but it's never been an issue.
Lately though it's been in the red, the least amout of disk space i've had is 4 something GB left of 62.2. I tried deleting programs, deleting restore points, disk cleanups and that sort of thing like everyone else has suggested and it seemed to work for a time, and then I ran a disk defrag and all the sudden more and more memory was disappearing each time i would refresh the window. I stopped the defrag, deleted some more stuff and it seems to be stable at 7.52. I was wondering what exactly I can get rid of to get more memeory (I am not very computer illiterate and am afraid of deleting files and programs I need...) and was wondering why in general my memory was filling each time i would refresh the page.
It's a dell inspiron 1525 with vista home basic if that helps.

Thanks so much.

First lets be sure that the memory is being used for legit purppses
Run a full Anti Virus Scan
Download and run a full and updated malwarebytes
How to Fix Shrinking Disk Space in Vista » My Digital Life
applicable to Vista
http://www.sevenforums.com/tutorials/11728-optimize-windows-7-a.html

My comp usually runs continuous scans with microsoft security essentials... Is that a good program to run, or do i need more virus protection?

First, I have added to my above post, after you posted.
I would use MSE, as you have, as welll as Malwarebytes. Scan weekly with Malwarebytes.

Thanks bundles, I just downloaded malwarebytes. We'll see how the scan goes.

The malwarebytes scan came back with only one adware thing... So I shall try some of the procedures on the pages you suggested, thank you again. any other suggestions would be readily welcomed.

If nothing else you are ahead of the game. Malware can be more than just annoying.

Thanks. I think now it's just going to be sorting through lots of unwanted programs and files. -_- I just wonder why the memory was being taken up during the disk defrag.

I think that the memory was used by backup and restore. I think that Before the defrag, back up and restore, backs up your files. Try turning it off before the next defrag and see the results. If you do turn if off you may lose the back ups already made, so be sure to back up your stuff.
http://www.vistax64.com/tutorials/163135-backup-status-configuration-enable-disable.html

I have found the defrag will end up using more space if you turn it off before it has finished.

I would add Super-Antispyware to richc46 post on security ran as per MalwareBytes (about once a week)

http://www.superantispyware.com/

The only reason I turned it off was because it was eating my memory and I was nearly out by then, and it had been running for about two or more hours already. I wasn't sure if that was normal or not.

Yes it can take a while sometimes depending on how much it has to do.
You can set it to start when you go to bed and it should be done when you wake next day.

And to add to all the above posts. Too many defrags is not good. Dont defrag on schedule, only when you have about 10% or more fragged on the HD.

where do you go to see how much is fragged? Like I said, my knowledge of computers is pretty sad.

What defrag are you using, the one that came with Vista?
If it is the defrag that came with vista. When you can begin to defrag there is another button that says analyze, click on that.

I think that's what I usually do, it doesn't really tell me anything except "this disk does not need defragged or that it does need it." Every other time i've tried to do it in the past it says it hasn't needed it, only the last time yesterday did it say it needed it. Would it be okay to try it again today, or should I wait?

Download and use the defrag that I have. It is better, quicker and it will give percentage.
Disk Defrag - Fast and Safe Defragmenter for Your Disks

I downloaded and ran the frag, and it cleaned it up, but it did use about half a gig of memory, as it was putting files or whatever it exactly moves around into my free space. Is that okay?

After the defrag, did you get the memory back?
Did you use the defrag that I suggested?

I used the defrag you suggested, yes. but no, i lost memory instead of gaining it...