I typically google to find the latest and greatest.
From there I get a baseline and if satisfied I do not let the program be a TSR. IMHO it is not worth the resource lost for that.
From time to time (every 6 months or so?) I'll fire it up again to do another check.
I also believe if you're running Vista, they'll have some gadgets to do it. Same concept though. What kind of PC you got? Home grown or name brand?
My '03 Sold.