changing ip addresses are not a problem. the ip addresses get reassigned every so often, when their 'leases' expire. you can read all about it at support.microsoft.com. if a pc happens to be off the network at the right moment, its ip address could be assigned to some other pc. it does not matter what matters is that all address are unique and dhcp server will see to that. if you find yourself configuring static addresses at workstation or other device for any reason (mail server, print server, wireless router)you are possibly headed for future trouble. you want dhcp server in charge of all addresses. you assign the address you need to the hardware you need to be at static address. otherwise you could get a situation where dhcp server tries to assign ip address which duplicates the static one you set up directly at the pc, see? other way folks handle it is to just save range of ip address for static assignments but i like dhcp server way because is sorta self documenting. if pc has non-unique ip address, it doesn't silently break anything-it just won't work.
bogged down network. check each computer for viruii (this sounds like Blaster), spyware, file sharing programs like kazaa, anti-virus autoupdates, instant messenger auto updates, real player auto updates etc. including windows Automatic Updates settings. if all hit 'get update' at once: clogged pipeline. look into proxy server? have a firewall? download automatic updates once to file server then over the network to several?
this could be Blaster loose on your network. unless you are keeping up your security patches and windows updates and virus definitions. with the dial-up connection, it gets harder to guess as it doesn't take so much to overwhelm that small pipe, maybe not Blaster, perhaps you've just added too many computers. or recently reconfigured Auto Updates or somebody's running kazaa... you don't say enough...
