Losing network connectivity

Please help me before I have a breakdown.

I have several PC's running Windows XP SP1 with every possible update
on the including the patch for TcpAckFrequency from Microsoft and am
having two major problems.

Firstly I have PC's intermitently losing network connectivity
(outbound). That is to say that they can not get onto the internet,
run IPCONFIG, browse the network properties (right click LAN) or get
into Event Viewer. Normally a reboot or simple log in and out will
rectify but the problem comes back even without a period of inactivity
so I guess stand-by modes are out.

Secondly I have 10 PC's similar in patches to above which connect to a
2003 server (PDC) and they have mapped drives to this server to access
shared documents. You can browse this folder in Network Neighbourhood
fine but try going into Word and File, Open change to the mapped drive
and it takes 4 minutes for the PC to respond and allow you to access
the file.

I have noticed on the Second problem that if you do a ctrl-alt-del and
go into Task Manager the Network Utilization is at 0% for 4 minutes
and then moves once the file eventually opens.

It is also worth pointing out on the first problem that when the PC
can not get out at all on the Internet it is still on the network (has
a mapped drive it can uilise) and also can be seen on the network and
browsed to by other PC's......

Any ideas apart from going back to Win98 or even Win 2000 would be
greatly appreciated.