How can I fix my computer? There are buffer/Norton Antivirus/Symantec problems among others...?

My laptop was working fine 2 Sundays ago, the next day, it wouldn't start. After that, it went just VERY slowly. I am constantly getting messages that "The Buffer is Overrun!" it also does not let me get onto sites like Yahoo or Facebook. The Google and Ask toolbar were deleted and I also cannot open... show more My laptop was working fine 2 Sundays ago, the next day, it wouldn't start. After that, it went just VERY slowly. I am constantly getting messages that "The Buffer is Overrun!" it also does not let me get onto sites like Yahoo or Facebook. The Google and Ask toolbar were deleted and I also cannot open itunes or upload pictures! How do I fix this? I already deleted the history, cookies and some programs thinking they may be the cause if a virus got in (I had mp3rocket).

I also HAD Norton Antivirus/Symantec, which the subscription expired for that day. Would that have an effect? It keeps telling me the main error is I need to update my subscription (which I don't want to, it's $40). However, if that's what it takes.

Thank you for any help regarding the situation!
Update: Wow, seems like general problem in the Norton! Thanks so much for responding quickly! If I could pick you all for best answer I would!
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