Have you gone back to your computer guy and asked him about what's happening?
If it is the wiring -- ethernet or something inside the computer, it may be an easy fix. The fact that your modem and router lights "go nuts" may be an indication of a significant download -- maybe a virus -- maybe just a clitch. Does it continue while you are using the computer -- or just when you start up?
I use Norton Antivirus and Security (2005) which has ad blockers and pop-up blockers options available. Not sure whether your antivirus program has anything like that. IE has it's own blockers that you can turn on and off (but there are still other IE blockers that I haven't figured out how to use).
If you do have toolbars and don't have a great need for them, remove them. They do slow things down.
I recently had my Dell desktop circa 2003 upgraded, cleaned etal and it is working reasonable well. I clean my temps and cookies regularly, but don't defrag the computer. I have more problems with my dial-up connection then anything else and that slows everything down.
I do download games from IWON (slow download), that have lots of ads (which slows everything down). I use the ad blocker on NIS to improve operations. I try to have 3 or 4 games going at the same time. When the PC had only 512mk operating memory, the PC would often lock up -- now that I have 4 times that amt, the PC doesn't lock up, but the dial-up download can't keep up.
I'm interested in anyone's response to computer problems since the issue is universal -- even it the systems are so different.