Desktop pc runs so slow even though it shows a lot of free space?

Not sure if this has been mentioned or not, but I was experiencing the same thing a few months ago.  I tried EVERYTHING...come to find out, we didn't have enough RAM.  Now that we've doubled the original RAM, it's SUPER FAST...no more locking up, etc.  Just a thought...hope you get it figured out!

download and run

http://www.cybertechhelp.com/download/file/vx2finder-for-windows-2000-and-xp

Download and run

 http://www.avast.com/index.html

Remove Norton anti virus,Stop sign, McFee Anti-virus before rebooting for advast, and turn off Window's firewall as Advast has one and double firewalling can cause a slow down.

Load Messanger and choose the don't acticvate at Start up, same for Quick time and Real Player.

Make sure your Flash Player,Directx, Java, and Quictime is up to date 

Download and install

http://www.safer-networking.org/en/download/index.html

make sure tea timer is on and you check for bad sites and block sites automatically

If it is what I think it is, Norton or Mcfee's "Friends" then your computer was already running faster as soon as Advast killed the 3 Trojans and 33 Malwares that usually are associated with NOrton and Mcfee.

Defraging is optional, but like re-formating the entire drive there are some risks involved, especially with older hard drives that may be just "barely holding on". Plus if the power goes out, if there is a brown out, or power surge during defragining your hard drive could end up just  "Fragged" Militarily speaking (But then again people that turn off their computers risk frying it everytime when powering it back up)

If you use your computer for high access hard drive applications like Gaming, media editing or movie copying then you might want to consider adding a secondary drive to the computer for such applications with the fastest access time which will keep the System files safe on the primary boot disk as Gaming and Media applications wear hard drives out faster. (Any profesional gaming/media machine has this  dual drive system, sometimes with the "gaming drive" bracketed as a portable to allow easy replacement when it fails)

I like doing an MSCONFIG check for startup memory being used. I'll run virusscan, ad-aware, and spybot (all free if you're using the free scan from McAfee). In addition, do a disk cleanup, soften the virtual memory your browser and downloading programs are using, then defragment. In addition, if you know what you're doing, you can clean out your registry manually, but I would never use a program found online to muck with my registry. That's just me. I find this routine is incredibly effective, and I can't recall the last virus/trojan/worm I've gotten since high school, even with frequent p2p'ing.

like the comment above said
MemTurbo works great too
if its not for some reason your startup

Just wondering, did you find a rogue program running?  If not, here’s what I do.  Maybe it’s my OCD, but I hate to wait on a cluttered, slow computer.  I want my PC as fast as possible, so I am meticulous about doing the following: using Firefox, removing spyware (w/Ad-Aware, Spybot S&D, Panda Platinum AV), running System Mechanic Pro6 or some similar program every once in a while (cleanup/defrag), backing up regularly with Acronis and external hard drives, and finally, about once a year, I reformat my system.  The reason?  Even though you may delete programs, empty your temp folder/files, and defrag, there’s still little chunks of ‘crap’ that remain.  Windows cleanup/remove utilities just doesn’t do a very good job.  Recently, I added MemTurbo4 (I learned of it from a post on this site), and it does a great job of freeing up a majority of my 2 gig of RAM.  This may all seem extreme to some, but it keep my system running fast!  Hope this helps.