Computers scan the entire network range when turned on. What is the reason and how to cure?

Question

Computer networks

A strange thing appeared on the network: at first, one PC began to scan the entire network automatically at boot, and since DHCP distributes a mask of 255.0.0.0, the range is large. Then another PC appeared with the same behavior. After that, the network is loaded and pings appear at the level of 800ms. Who can tell what the reason is and how to cure them without reinstalling the OS.
Note:
- OS: WinXP,
- connected services, cleaned autoload,
- checked on 3 antiviruses (cureit, Kaspersky Rescue Disk, ESET Endpoint) with updated databases - none found viruses
Addition:
I changed the mask to 255.255.255.128, then the PC quickly scans the range to the end three times in a row and then stops. But the next day the situation repeated itself.

Answer 1

first one pc started to scan the whole network automatically on boot

The answer is obvious - a new program (or update) has appeared. This means that it must be eliminated or blocked from accessing the network. Accordingly, you can reinstall the OS (each office should have a system recovery disk) or set up a firewall on your computer. Until you manage to localize the program, then it is impossible to offer something more specific.

Answer 2

- OS: WinXP,

Trojan. Heal the OS and put patches on it!
killer - \\live.sysinternals.com\tools\procexp.exe
boot blocker - \\live.sysinternals.com\tools\autoruns.exe
(copy and paste links into the explorer address bar)