Can't Access Certain Web Sites on Desktop but can on lap top

I have xp pro on the desktop at home and each time I log onto microsoft exchange server, it will not accept my log in or password. However, when I use my laptop, I can access the web site with no problem. Also, I have the same problem with my company's web site. It doesn't accept my name or password on my desk top but I can access it easily from my laptop. It seems to have started when I took the desk top in for some modem work. I brought it home and thats when the problem started. The place where I took it to has no idea why I can't access certain web sites. I disabled the pop up blocker but that didn't help. Any suggestions?