The biggest clue is the time of day. Let's assume you get home around 4pm.
Runs fine for a while then slows down. Hmmm After 4pm people are getting home from work and getting online. Students, already home, have already jumped online. Your ISP is loading up. Sites are getting loaded up as more and more uses go to the sites.
Things will slow down some.
Another thing is what DeathStar pointed out. Some programs will actively phone home to check on updates. Way to fix ones that do is to go in to the set up for the programs and either turn that feature off or set it to manual. AntiVirus is the exception to that rule.
Another one is you're visiting a lot of sites and picking up just boatloads of pure junk.
has an excellent program for getting rid of most of the crap. Hence its name Crap Cleaner. Also click the tools selection in the IE browser and select internet options. Delete files, clear cookies, and clear history are a must to run from time to time.
And the big one. AdAware Get that from www.filehippo.com
Make sure you keep that puppy updated. You have to start the update manually.
A clean system and a savy operator make for a smooth running rig.