I have seen resource issues with avast(free version)...specially on xp home machines..with minimal RAM....mostly just on startup
Avast! have just released a new version. I haven't had a chance to examine it yet so I can't say if that has improved or not.

I have it on a 2.66 core2duo with 1Gb of RAM, so I am not really expecting problems.

Most problems I see with systems...is lack of RAM.
I still come across xp systems with 512MB....
Same over here....................mostly ex-corporate machines that probably didn't have a full security suite on them. The new owner then goes and buys one of these full-blown security suites and wonders why the machine is slow.

My wife bought a machine from her workplace a few weeks ago: Pentium core2duo, XP pro SP2 (!!!!) and.................you guessed it! 512Mb of DDR1 RAM

So I stuck in a 500Gb SATA drive and 512Mb of RAM then applied SP3. It made quite a difference. One thing that struck me was that it only had a 40Gb drive in it (I didn't know that they made SATA drives that small), so it must have been part of a network, possibly a thin client environment?

It came with Avast! on it................and I know that hundreds of these things have been unleashed on the unsuspecting East Yorkshire public
Thanks for the heads up MLF

I have also seen configuration problems with security software, particularly the free versions, which tend not to give you many options. I am mainly thinking of gaming here.

I personally don't use "scan system on startup" or "scan file on closing" as I really don't see them catching much that scanning before shutting down and scanning a file on opening won't.

With a modern computer I really don't see resources as an issue; if you are having problems it is more likely to be software incompatibility and conflicts.