As for the OS and things to check, nearly all are routinely done. And I simply don't muck the OS up with redundant app's or conflicting progs (50% free resources), plus I gave up on patches soon after SP2 because of the Doctorate's in IT needed to determine what's needed vs. not, so the system's pretty sleek and not too mucked by MS molassis.
As for patches, my AV runs a prog weekly that notes any un-updated patches and/or vulnerabilities, then sets up firewall rules to protect against those things (much better than endless hrs deciding what patches to install and totally better than what happens when an un-needed patch slips by, not to mention the nightmares of ruined OS's when trying to unistall MS patches.
But for you advice...
update and run your antispyware (weekly)
update and run your antivirus (daily)
run chkdsk /r on your drive(s) (about every month or so)
delete all the files found in \windows\prefetch [they will rebuild themselves] (done every week or two)
run disk cleanup off the system tools menu (done every other week or so, I even run registry booster about every one or two months, now that they finally enabled hits to be fully read)
run defrag off the same menu (done even if analysis says not req'd, when seeing note worthy chunks of red)
additonally you can use msconfig to come up without loading a lot of other programs upon startup. (done that long ago and every couple of weeks I check that to ensure new stuff's not there and things are as I left them)
second look at your nics connection speed (N/A, and ethernet cable is proper and good, and apparrently also good is the line filter)
run ping tests between your gateway ip, your dns ip and the website. (No idea how, I'll drill)
run a tracert to the web site in question. (seems I need to drill that, not fam w/it)
I think if the RAM's not the issue, and neither is the CPU (as it clock out fine and cooling too), could our illustrious leader's 911 spyware be causing it (I work sometimes for the DoD and sometimes mildly classified oveseas projects).
I just dunno, I've battled it til I'm blue in the face. Only thing I havn't done is the HJT regimental cruisade.
Regards and hap-e-trails, Steve Hopper