Defraging is a good idea regardless, but if that were an issue, I suspect you'd likely see a noticable slowdown on the entire computer and not just with the net. But those are the easy suggestions, so good to start with them. If defrag and cache/temp file/cookie clearing doesn't work, maybe this will help.
Aside from pics and vids loading, does the rest of the page load up quickly (I.e. the bacgrounds, buttons, text, etc)? Check your modem for traffic use. If you can't monitor traffic, you can get
Zone Alarm Firewall free (it's what I use with no issues in years) and it will display traffic in the task bar. If you notice a lot of net traffic with no browsers open/loading, no downloads running, messengers logged off, etc., then it's likely some sort of malware tying up your net.
Zone Alarm is good about blocking suspicious activity, so malware won't get in or out if that's your problem, but wont rid the system of the infection. AVG and Spybot S&D are my two usual go to programs for clearing infections. Since you said you already tried checking that stuff, do you see any other odd processes in your task manager process list (especially if they are using CPU time when you aren't expecting any activity on your system)? If so, check out
THIS SITE. It lists most every known task, including malware, that could be running on the process list. It's easy to search and will let you know what the process is supposed to be used for, and if it's possible malware, how you may be able to get rid of it. Heck, I've used it to see what kind of unnecessary programs I've had running at start up just as a way to do regular system maintence.
Best of luck.
