Cannot access some web pages

Until about 2 months ago my friend's Win 7 Home Premium 64 bit laptop was working fine with a wireless connection to an Edimax home router. Then he started to get problems accessing some web pages - note he can generally access the sites but some pages will not load - eventually IE8 or FF4 will timeout and the troubleshooter will engage and eventually say it cannot find a problem.

One such site is www.hpb.co.uk where he can get the home page up but when he enters his logon info it just sits there and looks at him. Another strange happening is with Hotmail where, going through the web interface, he can access his inbox perfectly but if he tries to access the Calendar or his Profile the page just sits there again. The Windows Live 2011 Mail client cannot connect and comes up with error code 0x80048820 but none of the solutions I can find for this code seem to resolve the problem (neither do they explain exactly what this return code means, but that is another issue).

We don't think it is the router that is the problem because we have had a replacement one from the vendor and it gives the same problem. We installed FF4 after the problems started but it immediately suffered from the same problem.

We have tried resetting the DNS Cache, the IE settings - we've cleared out all the IE temp files - once after doing this the problem went away for a couple of days! but further attempts do not get us working again.

Any thoughts?

Thanks for your interest and suggestions.

We have run Ccleaner and that is what appeared to get us back on the air the first time around, but when the problme came up again 3 days later, running it for a second time did not change anything.

We downloaded and ran Firefox 4 after the problem started but we got the exact same issues.

We have run Microsoft Security Essentials and it gives the system a clean bill of health. We haven't run malwarebytes but I'm not sure what that will do over and above MSE - any suggestions?

We haven't tried a new Hotmail account but we have tried a new userid/password for the www.landsend.co.uk site which is another one that gives us trouble and we get the same response, so I don't think its worth setting up a new Hotmail account?

I'm wondering whether we have some sort of MTU size issue between Win 7 and the router, but I believe Win 7 is supposed to automatically adjust in this area.

Ah, OK. Didn't pick up on the Windows User account - that's an idea - I'll give that a try in the morning.

We tried the compatibility button but that didn't change anything and we only have one firewall on the system the - default Win 7 one.

I think we fixed the problem. It seems the MTU value in the Edimax router was set at 1492 and changing it to 1500 resolved the problem. I tracked it down by taking a Wireshark trace of the line and saw corrupted messages and duplicate ACKs. A value of 1500 of course matches up with the value that Win 7 uses in the PC. How the value of 1492 got into the router and survived both a reload of the firmware and a reset to factory defaults beats me, but changing this value did fix the problem! Thanks everyone for their suggestions.