Troubles with PING, TRACERT and time synchronization in WinXP SP3

Hi,

I have a home network consisting of a couple of PCs connected to a D-Link
DIR-635 router, both via cable and WiFi. Among them there is an old PC
running Win98SE and two newer PCs running Windows XP.

Everything worked well untill I installed SP3 on two WinXP computers.

The old Win98SE machine can send ping, execute traceroute and synchronize
time with a help of third party utilities. No troubles.

The WinXP computers give me headache.

1. None of them is able to synchronize time with time servers, no matter
whether I use the built-in Windows functionality or third party utilities.
Please don't give me advice to try yet another time server... because I tried
a real lot of them and tested them on the other Win98SE computer. They did
work well. I also tried whatever solution I could find on the Internet,
including reregistration of w32tm.exe.

2. The router cannot be a problem because, as I said, the Win98SE machine
connected to the same router is able to synchronize time.

3. What more, even if I try to DIRECTLY connect WinXP machines to the
network (without any router), they still are unable to synchronize time!

4. The firewall, antivirus, antispyware etc. are not guilty either. I
stopped all of them.

5. None of the two WinXP machines is able to PING computers outside my home
network (ping always fails). They cannot ping even when directly connected to
the net.
However they are able to ping each other within my home network.

6. None of the WinXP machines can run TRACERT properly, it always fails,
even if the PC is directly connected to the network.

Perhaps I am mistaken - but everything points to the Service Pack 3 as the
culprit.
Can anybody help?

Kind regards
mjw

Anybody can give me a clue?