My network completely collapsed for no good reason after taking the client machine apart and putting it back together exactly the same way. It worked perfectly before that.

In trying to set it back up, nothing worked, and I found it easier to uninstall ALL protocols and services, (apart from TCP/IP), and network adapters from both machines, and re-installing them to give myself a clean slate to work from.

OK, with that done, the folowing protocols are installed...

Client for MS networks
QOS packet scheduler
File and printer sharing for MS networks
Internet Protocol (TCP/IP)
NetBios
IPX/SPX/Netbios compatible transport protocol.

The last two are installed purely to support a game or two that seems to prefer networking with them but otherwise aren't there for anything.

Windows firewall was then turned OFF on both machines, and the remaining firewall on "Server" (Sygate personal pro) was set to allow all, rendering it harmless.

OK, First the network setup wizard was run on the internet machine ("Server"). It was then run on the client machine ("client")

Both machines can ping each other on the network with no problems. A portscan of client helped me find out I could telnet succesfully to port 17 (qotd) of client, which I did because I was bored. No problems there then!

The network MSHOME, on both machines, shows that both "Client" - and "Server" - are connected

"Client" has an internet connection using "server" as the gateway, and can access both it's own shares, and "server"'s shares.

"server" can access it's own shares, but CAN'T access the shares on "client"

the attached file contains the IPCONFIG/ALL readouts of both machines. It also contains a network connection on "server" that I set up as an experiment, but now reappears each time the computer is booted, nomatter how many times I delete it.

Any help to resolve this issue would be greatly appreciated.

Change your attachment IMMEDIATELY. Never publish your ISP's IP-address!
And also don't publish your NIC's MAC-address (see my xx-xx-xx etc. below)

Using a router and fixed IPs, my ipconfig /all looks as follows:

Windows 2000 IP Configuration

Host Name . . . . . . . . . . . . .: pc's name
Primary DNS Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Broadcast
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Realtek RTL8139(A) PCI Fast Ethernet Adapter
Physical Address. . . . . . . . . : xx-xx-xx-xx-xx-xx
DHCP Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 192.168.1.101 (my PC's IP)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.254 (my router's IP)
DNS Servers . . . . . . . . . . . : 192.168.1.254 (ditto)

All my other PCs are the same except for their own IP-address.

In your attachment I noticed incongruencies between the 2 setups:
Client has DHCP enabled, Server not
Both have IP Routing and Wins Proxy enabled, this should be DISabled.
Server has no Default Gateway.

I think I gave you this link already, but look again:
Have a look at the attachment here:
Basics-networking

PML! Thanks RBS! sometimes my stupidity knows no bounds. I should know better than to publish my MAC adresses on the web. The ISP's IP is pretty dynamic though. It changes quite a lot. I do see the point though.

I will have another look at that link, and pay closer attention to the attachment. At the moment, nothings changed from by original post network wise.

OK, I take it back. I hadn't seen that post before. Slightly unrelated fto it though, it got me thinking about manual configuration. Can Internet Connection Sharing be turned on and set up manually? Is it as easy as sharing your connection as you would a folder, and setting the ip's correctly?

I don't know where that is done in XP, but it should be a very straightforward simple setup in your Settings/Network. The disadvantage is, that the PC that is connected to the web, must be always switched on, if the other PC wants to go to the web.
Get a router, it protects your PCs better and makes network life a whole lot simpler.