Have you tried turning your anti-virus/firewall/other security software off and leaving your computer doing nothing for half an hour or so? Sometimes anti-virus programs, especially in conjunction with certain other programs, can cause enormous amounts of lag. One such example is Norton Anti-Virus 2006 and Zone Alarm Pro 2007. Put them together and you'll go nowhere for half an hour. So I'd try turning off your anti-virus, firewall, all the security software, completely, leaving it alone for an hour or so, and seeing if it still is really slow when you come back.
If the lag disappears, it's time to switch anti-virus and/or firewall.
Also, what's the hard drive light doing when it's running really slow and crashing? If the hard drive light is always on, that's another common cause of lag, and may well be caused by security software.