<sigh>
System Idle Process (SIP) just tells you how much free CPU process you have, nothing else. When you're running programs and games, they use up CPU processes and you see SIP's value drop.

Go to http://www.kaspersky.com and run their online scan over your system to get a second (better) opinion on Norton's scan results.

As regards adware and spyware, I'd suggest running more than just Spybot. At the very least download Lavasoft's AdAware (it's free) and even another - I'd recommend XoftSpy but others may suggest Spyware Doctor or other.
Did you get Spybot Search & Destroy from HERE? If not, you may have a rogue anti-spyware program there so download it from that site.

Always update anti-spyware and anti-virus program detection bases before scanning your system, even if you've just downloaded them.

After all this run hijackthis, scan your system, save a logfile and post it here. Don't run hijackthis without doing the rest as they will most likely eliminate a 'nasty' or two and save us some pissing around.

truetejas is absolutely right - if your system was OK before with the RAM you have, that won't be the problem.
Unless you've installed Limewire (a horrible resource hog IMO) it's down to something nasty.