Five (plus one) Quick Ways to Speed Up a Slow Computer

Add Memory.  Different flavors of Microsoft Windows have different memory capacities, and this is further compounded by the processor architecture (32 or 64 bit).  Check the chart linked here and see if you can add memory.  Doing so can definitely speed up a slow computer.

Go Wired.  Wireless networking is convenient.  It's more fun to surf the web and do work in your living room.  That's where I'm writing this, after all.  But if your computer is slow, the culprit might just be a slow wireless connection.  If this is the case, try using an Ethernet cable instead of wireless.  If you're very far from your router or modem, try an "Ethernet over power" solution like the one mentioned in this article.

Defrag.  Your hard drive stores information wherever it can, not contiguously.  Deleting data leave holes, which leads to files being saved in pieces all over the hard drive.  This can lead to slower file access and, of course, a slower computer.  Defrag your hard drive regularly to consolidate data and make access faster.

Reboot.  It's healthier to leave your computer on at all times -- cold startup is tough on hardware and helpdesk friends tell me 80% of the time, that's when hardware fails.  That being said, you do need to reboot once in a while, especially if your computer is running slower than usual.  Rebooting will clear out your pagefile and virtual memory and restart background programs, "freshening up" your computer for faster performance.

Spring Cleaning.  Take some time to clean your computer once in a while.  Uninstall programs you don't need and take any programs out of startup that don't need to run in the background.  This article mentions how to run MSCONFIG.  If you're going that route, definitely triple check every app before turning it off.  That being said, shutting down unnecessary background programs and uninstalling software you don't need on a regular basis are great ways to speed up a slow computer.

Bonus Tip!  I don't know how many times I've seen clients who have two or even three antivirus programs running at once.  Antivirus programs aren't like locks on your house.  You only need one good one.  More than that and you'll definitely have a slow computer.