It sounds to me like you have excessive load on your system resources. If your computer is locking up at the same point all the time or you are getting memory failed errors, then yes, your RAM might be bad.
Otherwise, I would check your system resources and see what is using them.
Hit CTRL-ALT-DEL and click Task Manager, then click the processes tab. Click the CPU heading to sort by CPU use. This will give you an idea of what programs are running and actually using system resources, and how much memory they are using. Next, click on the Performance tab. This will track the CPU use over time, and your page file use. Normal CPU use with an email client, web browser and one or two other programs like Yahoo Messenger will give you 0-5% use. If you are streaming music or playing music, the CU use will be a bit higher.
Now, should something show up on the Processes list as using lots of CPU, go to www.google.com and type the name, like explorer.exe, into the search box. This will usually let you know what the program is and you can decide if you have a problem or not.
I next remove any programs that you don't need, and anything that starts automatically when you start your computer. Most programs have a preferences setting that will remove the "Load on startup" behavior.
Make sure you have a good anti-virus, I personally suggest F-secure, and make sure that you have defragmented your hard drive recently.
Formatting and re-installing the system is something you shouldn't need to do unless you have serious problems, and is only a last-ditch choice. It also requires you to re-load all your programs and you will lose all your data unless you have everything saved elsewhere.