If you are really that security paranoid, you can have these AVs running at the same time(Real Time):- (provided it is Win XP SP2--that is my OS)

-Kaspersky Personal Pro (Not so sure about Personal Edition)
-ESET NOD32
-F-PROT (THIS ONE IS LOUSY,scanning time is crappy and when I was testing it, it can't even detect EICAR "virus" =.=...lame)

But then again, as a word of caution, having two AVs in Real-Time Mode is a speed disaster, ur rig will lag for sure.

As in having two AVs will clash IS NOT the case ALL the time. I used Norton System Works 2006(The Anti-Virus inside the set, I mean) with NOD32, but my system was stable.(perfect rather,lol, but slow) It depends on which one is SOLEly for scanning, which one is for Real Time Monitoring. You must not get that wrong, else... I rest my case.

btw, Spybot S&D is for killing spyware and adware, it is not a AV. (plz take note)

A word of advice, try to check whether ceratin AV combos can cause catastrophe... From my experience, NOD32, Kaspersky Personal Pro and F-PROT are AV's u can mix. Definitely no Problem. I tried the combo myself.

AVG and Norton, it really eats up ur memory, how much ram u hav ( juz wondering)? I might suggect u knock out Norton ler, not much use 1. I have Norton reporting hundreds of viruses, but never getting around to removing half of them. =.=

NOD 32 also has RTM(Real Time Monitoring), Kaspersky also has it, Avast,AVG and Antivir also has it. It is just that we need to explore other options too. No offence, but Norton ought to be thrown out, try meddling with other AVs. You might like a "change of scene". Good Luck.