SP3 wont install?

My computer won't update to service pack 3....anyone have any ideas on why this might be?

I have AVG free..so I shouldn't have any viruses...although..I might cause some websites don't load up on my computer...such as toshiba.com(it works on other computers on my home network..just not mine)

I don't know what to do...the little "updates are ready" thing is annoying..and websites not loading is even more annoying...

so if anyone has any ideas on how I can fix this..please help

If you need more info..just ask..but tell me how I would get the info
:)

thank you