Freezing, locking up, lagging.

I couldn't actually figure it out which one out of those three happens, I will try to explain as best as I can.

I get totally random lock ups. Happens during a game, watching a movie, browsing or just listening to music. Sometimes it lasts for a minute, sometimes 90 secs or so. But it's not a total lock up, I can move the cursor, click on things, even though clicking doesn't open anything during the freeze, however what I clicked on opens after lock up ends, as if it's lagging.

For example, whilst I'm browsing using mozilla firefox, I click on "open a new tab" then PC locks up and mozilla doesn't respond, I can open, say, msn messenger and type something to a friend, I can type but it is very laggy, I can click on a mp3 file but it doesn't start until the lock up seizure ends.

As the lock up starts, that HDD activity light on the PC case (normally blinking) lights up continuously without blinking, as if hdd using 100% capacity and computer can't handle anything else.

I did various tests for hdd, ram, graphic card, I did 3 formats and re-installs, virus scans with different programs, checked temperatures but couldn't find the cause of this problem.

Any help that could put me on the right track to figure this out would be much appreciated, it's been more than 3 weeks I'm putting up with this problem and I'm at the edge of throwing whole case out of the window.

specs:
win7 home edition 64bit
i7 920
hd5870
4gb ram
1tb wd caviar green hdd
ex-58 ud3r gigbyte

Hi Ereldian,
let's start with the basics. Do you know if your system is running with the latest drivers, Bios and firmware? If not then please update as necessary.
Also are you running any security apps? If not, this is one of the best ( http://www.microsoft.com/security_essentials/ )
If you need any help tracking down either Bios or drivers then please let me know.