Please guys help me out on my network problem.

I have a cisco router 3640 series with ios 12.1 (5) T... just recently when i started experiencing a slowdown on the network when browsing the net... I troubleshooted the network to see if there is a problem of virus or network card... I thought the first time was my serial interface card connecting the network to my gateway (provider)... even planed purchasing new cards (wic-1t+1e2w)...

A friend adviced me to use bandwidth monitor and monitor the bandwidth utlization... percentage of my utilization is less than the packet loss with about 20%... the packet loss is about 100% while response time is just 80% or so.

Please, what do you this is responsible to the loss of packet?
Is the vulnerability on the product i.e. cisco 3640 series router?
If yes, I need your help guys on what to do

Thanks