unreachable sites

[idr]

my os is win98se memory 520mb, ie6
some of the sites are unreachable for me already several days.
I can reach them from my friends computer but my own doesn't accept them. for the time being I couldn"t open only two sites, one of them Adobe.com, other can be reached without a problem. my Internet Options settings are all on default, as they were always.
I delete the temp files and cookies daily, and today I checked my computer with two spyware programs, but it didn"t help.
the error message is always: this page cannot be displayed.
some days ago I opened the adobe site without problem and in the meantime I didn"t change anything in my settings.

please advise what can be the reason for it and how to rsolve this
thanks