Question : Subnet Mask changes when restarting computer

This problem has occured on two windows 98 and windows ME computers at different branches at my company.  Our internal ip addy is 192.168.1.XXX and our subnet mask is 255.255.255.0.  What happens is when they shut down there PC at night and come in the morning there subnet mask changes to 255.0.0.0 therefore they cannot get on our network drive everything else seems to work like internet and email.  We are running dhcp and I have copied the settings on a different computer that doesn't have this problem over to the troubled computers but still nothing.  Lately I would have to just tell them to release the IP and renew it then it seems to work for a few days.  

Answer : Subnet Mask changes when restarting computer

id start with reinstalling TCPIP on the workstations (remove everything out of networking), reboot and reinstall the lot, wont take that long.

also in dhcp do the 2 computers having the problems have reservsations?

also is there anything on the network that could be issuing them another ip address (i.e. another dhcp server)

rememeber DHCP servers can be:-  
wireless access points
networking products
some ADSL routers
windows xp (with internwet connection sharing / 2000)
some printers do.