*Floats in on a magic carpet stolen from TickleTheater* I recognize what is happening to you, for it once happened to me. However, the possible causes are numerous. Firstly, I second Missy Val and the esteemed Count; grab Firefox. Not only is it a good idea regardless of whether you have issues or not, but it is sure to speed up your browsing a little bit (Although Opera has been shown to be faster on Windows machines, but not as good at rendering pages correctly).
Secondly, it could indeed be an ISP problem. Tell me, when downloading files, is your download speed fast? i.e., when you save a large file and the download window appears (In Internet Explorer), what speed does it show? 176 kb/s, 52 kb/s, etc? Going along with that, who is your ISP and what level of internet do you purchase from them? What speed do they say you should have? If you tell me your ISP name, I can find that number. If you are downloading files at the rate you are supposed to be downloading them, then it is not your ISP, more than likely.
Thirdly, you could very well have spyware and adware on your computer. See this thread:
http://www.tickletheater.com/showthread.php?t=13644. Grab and install Ad-Aware, Spybot, and Spywareblaster. Scan with the programs that do scanning and remove anything they find. If you don't have a firewall or antivirus program, grab one of what you need (Suggestions are in that thread). If after doing all that you are still running slowly, let me know and we can dig deeper.