Hi, there:

Both my IE 6 and firefox switch automatically to reg.popo.163.com and show Apache
test page,saying that Apache has been successfully installed. Any
other website typed in the address bar give me the same page, although
the address typed in may remain intacted. That means, I cannot use web
browser to navigate internet at all!!!

I have tried to reinstall IE6 and firefox, it just does not work out.
Norton and other virus cleaning software show no infection of virus
after a complete system scan (with system restore off) according to
the up-to-date virus defination file.

Any help and comments are greatly appreciated. By the way, I am using
XP prefessional.

I was using norton and tried alot of different kind of antivirus but
when I use Nod32 I was really shocked because my Pc was full of
hundreds of infections.

Try nod32 please you can use it 30 days for free trial with all functions.
its probably a virus or trojan I am afraid your PC is infected.Make a
scane with Safemode.
http://www.nod32.com/download/download.htm

you can have some free antivirus antispy programs here.
http://www.download.com/

Hope this will help you 

Note : Norton and other virus cleaning software show no infection of virus
after a complete system scan (with system restore off) according to
the up-to-date virus defination file. BE sure not to work with 2
antivius programs at the sametime.