"Do I first run the netgear disk on my desktop and make a wired connection (which is fine, the desktop doesn't need to be wireless)..."
Yes, insert and run the disk, it will guide you through, step by step, of what to plug in and WHEN to plug it in and what settings to enter. You'll need your Talktalk Welcome booklet if you can't remember your username and password.
"......then run the netgear disk on my laptop and make a wireless connection? Does this mean I will, whilst setting the network up, have to first plug the netgear modem into the desktop, then the laptop?"
No, you don't need to do anything with the laptop, cas5090, other than make sure that's it's built-in wireless adapter is active, check your laptop's manual for that, but usually it's a matter of flicking a switch or pressing the Fn key and some other key, on the laptop's keyboard. The laptop will be connecting wirelessly to the router, so during the Desktop's setup (with the disk) make sure you set up the wireless option, you'll see it when installing the router. You'll be asked to set up wireless security and set up a password for that wireless security. Choose the WPA-PSK level of security.
When wireless has been set up, turn on the laptop and it should automatically detect the wireless signal from the router and ask you to enter the wireless security password you created, do that and click connect and you should be surfing on the laptop.
"As my son will need the internet for revision this evening, is there some way of saving my current settings with the speedtouch modem in case the networking goes wrong? if so, where will i find these setting?"
You should still be able to use the modem in the normal way, cas5090, installing the router wont stop you doing that. It's just that you can't have them both operating on the same line at the same time, so just switch off the modem while you're using the modem to connect. You, obviously, wont be able to use laptop's wireless option to surf while using the modem as the router will be switched off.