Win 7 - awfully slow internet

Hello there,

I am running Win 7 64 bit.
About a week ago my internet began all of a sudden to have problems. First I thought it was a virus but I went through all the steps of cleaning up my pc. The problem still remained.
What I am experiencing are the following:

1. pages do not load or load very slowly
2. thumbnails don't always appear
3. sometimes I get a very basic view of a page (without the graphics)
4. this message appears: cannot connect to primary DNS server
5. sometimes need to refresh page a few times until it loads
6. downloads are very slow

I have talked to my ISP and together we checked the line (connected a different pc and it worked just fine).

I really need the internet for my work. Please help me.