There's not a lot to it. Making sure that no services/programs that don't need to be there are loading on startup. A lightweight AV/other software also goes a long way. Eg, most browsers are obese these days. Even Chrome. Something like K-Meleon would be wonderful if it looked a little better.
So many people have installs of XP (partially because it's older, partially because it's less secure than Vista/7) that run like crap because they have all sorts of mess installed. Don't get me started on Virus infections. "But it's okay, I was using Norton. My uncle's vet says it's really good because it came with their computer." Wrong. Many of the popular AVs, in practise, are useless. Norton, McAfee, Trend, all bloated horrible crap. I'd also like to point out that CA Antivirus is potentially the most disgusting piece of software I have ever seen running. MSE and Avira are very light and effective. Oh, and they're both free and more effective than the big two.
That reminds me. OEM preinstalled software is usually bloatware. Toshiba, Sony, HP, MSI are the biggest culprits of this. Lenovo, Dell and Asus generally aren't too bad. Although Asus is hit and miss.
Printer "helpers" are a big one. They sit in the tray and suck memory for the once a month that you might scan something in without doing it the proper way (via the Printers and Scanner menu). This also only applies to Vista and 7, but the desktop gadgets are horribly bloated. Google's are even worse. Don't just no choose any, disable them.