Previously deleted files piling up

Say I only have like 25 GB's of disk space, and I download a lot of large files pretty often, ( like 4 GB's or so ) could this cause any damage over time by by consuming space somewhere else after I've deleted said files? From what I've heard, for the most part, most files aren't ever completely deleted, so I was wondering if all these traces of previously deleted files on my HD could possibly do any damage by consuming space in some other place. ( File Nirvana? )

Sorry if I worded that poorly... Anyway, basically all I'm wondering is provided I delete the files and now have space free on my HD, do I have to worry about all of these traces of previously deleted files adding up and do some kind of damage?

I think the only worry that may be related is over long periods that files that your using may be fragmented causing the disc to overwork. Make sure you have a decent defragment app and run it on a regular basis though some say to much isnt good. I guess about once a month or so although others may know more on that than me. Most will also tell you the Windows defragmentor is crap which seems the case.. I currently use Diskeeper since it has an automatic feature as Set It And Forget It which makes it simple. As for deleted files they are just wtitten over as new data arrives so I cant think of any other worries unless your consorned about others being able to view the data thats been deleted which is a different subject...

All right, I hate to admit to this, but I've never actually defragged.

After I've download Diskeeper or some other piece of defragmenting software, ( which I will do since you say the Windows defragmentor is crap ) is everything pretty self-explainitory? Is there anything at all I should know?

try mst defrag its pretty good, it actively defrags, as in when files are created, it managed em, by not allowing to defrag...
it works for me....

just use O&O defrag software and u will have no problem what so ever :)