Tried turning it off and turning it on again?
Defragged the hard drive?
Ran a sweep on your Anti-Virus & AntiSpyware?

If it's still rubbish... here are some possible problems/solutions:

1) Your computer is crap.

Is it older than 5 years old? If you don't feel up to reinstalling Windows, then it's probably easier to buy a less crap one. Just bought it new? Take it back and ask whoever you bought it from.

2) Your computer is full of crap.

Stop browsing on dodgy sites and installing crap. Google every program before you install it. Do Ctrl+Alt+Del shortly after booting up when you get a steady desktop. Then fire up your programs as normal. What are the Page File & CPU Usage graphs like? What are the Mem Usage figures? Anything looking awry? For what programs? Sort your processes by CPU. One of those processes will be hogging all your cpu power. Anything taking up more than 20% of CPU (or more than a couple 100k memory) should be immediately turned right off (right click > End Process) unless it has 'SYSTEM' as the username. Leave these ones be as they are usually deemed to be important for Windows continued functionality.

3) Your internet connection is crap.

On Wi-Fi? Then try it with a cable. OK now? No? Speak to your ISP. Yes? Turn your modem, router and computer off. Wait five minutes. Then turn them back on in that order, waiting a minute or two for the lights to stop flashing on one box before you turn the next one on. Any good? Yes? Great! No? Consult your manual and try setting up your WiFi again.

4) Other.

Last gasp: download CCleaner and use it to empty all your temp folders & internet cache, run a fix on your registry, and also browse and consider cutting back on the startup programs? Or just ignore it and return to the top of the list when it annoys you too much.