IP address getting modified

I am new here and think I read all the related posts, however can not find exactly the right answer.

I have a network with 4 computers, 1 network printer, a linksys router, and a speed stream DSL modem.

Two of the computers are running Windows XP Professional Service Pack 1, one computer is running Windows XP Home Service Pack 1, one computer running Windows 98SE.

All Systems have the latest updates from microsoft, The anti-virus software is Panda Software's Platium version 7 with current updates.

The computer with the problem is configured as follows: AMD Athlon XP 2400+, 512 MB PC2100 Ram, 80 GB Hard Drive, Realtec Lan. etc.

The systems were working correctly Saturday, however today one computer can not find the network printer who has IP address 192.168.1.103. I am unable to ping any system on the network or the router. All other systems can access the internet and printer - this one can not. I am required by the ISP to have ip set at dynamic not static.
However I tried setting a static IP of 192.168.1.105 subnet mask 255.255.255.0 something is changing that. Could it be a virus, spyware or something?
I have run Spy-Bot, Spyware Blaster, CWShreader and they found nothing out of the ordinary. Hijackthis found some things but I can not include what it found because I can not access the system from any point on the network and it is in a different room than the other systems.

The computer had several viruses it picked up from e-mail but all have been found and disinfected or removed.

The IP address that keeps getting set to 192.168.254.127 or 192.168.1.127 it is normally 192.168.1.100 to 107 depending which order the computers are booted.

Thank you for any suggestions.

 

Most DHCP problems are cause by having 2 DHCP servers running on the network. Shut down all computers and the boot the afflicted one and see if that doesn't solve the problem. If it works then you have to find the other dhcp server. You can do an ipconfig /all on the aflicted machine when it has gets an incorrect address and it should say where it got the address from. Someone may have turned on internet connection sharing by accident on one of the other machines.

You mentioned that your ISP requires a static IP address. They are only concerned about the WAN IP on your router which should be configured however they say. Post back and I will help more.