Problem Accessing Frequently Visited Web Sites

I have a customer who is having trouble accessing frequently visited web sites.  She has a cable modem connection that is in proper working order on an IBM Aptiva running Windows 98 and IE 6.0.  Until recently, she experienced no problems.  However, recently she is having trouble accessing web sites that she visits on a regular basis.  Sites she doesn't use a lot work no problem, but frequent sites time out as if they are down.  These sites can be accessed from other systems on their network.  I have tried upgrading her browser, clearing the cache and cookies, purging and rebuilding the TCP/IP stack, upgrading her NIC drivers, disabling all proxy server settings and running Ad-aware to remove spyware (of which there was lots on their system.)  Nothing has provided a permanent solution.  Short of reloading her machine, I do not know what else could be causing her problem.  Has anyone else experienced this and is there a way to correct it?  Thanks!