have you checked for spyware & viruses? what is the specs of your pc as in cpu speed, ram amount, hd capacity & free space, windows version as in win98, winme, win2k, xp & vista?
You should be able to get all that information by right clicking on your "My Computer" icon. If not you can use an program such as Belarc Advisor (http://belarc.com/free_download.html) or PC Wizard (http://www.cpuid.com/pcwizard.php). Personally, I think you must of contracted a virus or spyware. If your computer has been running fine until today that is most likely the reason. What anti-virus program do you use and do you have any Anti-Spyware programs?
It could be possible your computer is infected with alot of spyware and other unwanted programs. I suggest try running Hitman Pro. This is a hot piece of Dutch program, which installs the best anti-spyware tools. But I suggest adjust the configuration. Only allow these programs to be runned: Ad-Aware Personal CWShredder Spybot - Search and Destroy. All the other programs and possibilities should be turned of. Because the rest is payed software. After the program finished scanning your comp should run much faster.
Hopefully, you may even need a better Anti-Virus program as well. I'd suggest AVG AntiVirus free edition or AntiVir Personal.
Back up your data and format your HDD. It's the best way of getting rid of unwanted stuff. Then after you reinstall windows, get yourself some good anti-Virus software and a spyware blocker.
