I use Outpost 2009 (free) on Thinkpad / XP Pro SP3 with IE8 brower.
I also have avast running alongside MSE beta and so far all good. I have posted about this.
I work Outpost with combo - runonce / learn mode - to set basic rules default and so far all good.
I use OA (pay) on 2 computers with profile below - Firefox browser - and so far all good.
Also OA (pay) on Asus P4 with much same profile as computers below - so far all good.
I don't think I push these computers too hard as most my time is taken up just getting through my work routine. And doing a bit of research whenever I have time spare time. Or else repair. maintenance, build of computers. So pretty much stay on safe ground - just pesky adware sometimes.
I use ASK toolbar for research purposes - but only for record / convert and instead search web using more conventional tools, mainly google search box, but also academic - like University, Tech Institute or W3C - search engines. So I guess that cuts down a lot of the likelihood of malware intrusion.
Now firewalls settled in, I cannot tell difference Outpost and OA with general running. So far.