My copy of IE 7 just stopped working. Just like that. This precipitated a general reinstall which was a bit overdue anyway but anyone any ideas? Email and FireFox worked (when installed as a temporary fix) without problems.
I reset all the security settings to default - no improvement.
The reinstall cleared the error but a friend has just called me to say the same has happened to him - might this have been caused by a dodgy windows update?
Kaspersky found nothing and a HiJackThis scan was clear.

How to use Reset Internet Explorer Settings (RIES)

To use RIES in Internet Explorer 7, follow these steps:

1. Click the Tools menu, and then click Internet Options.
2. On the Advanced tab, click Reset.
3. In the Reset Internet Explorer Settings dialog box, click Reset.
4. When Internet Explorer 7 finishes restoring the default settings, click Close, and then click OK two times.
5. Close Internet Explorer 7. The changes take effect the next time that you open Internet Explorer 7.

Note for users who cannot start Internet Explorer 7 for some reason, use RIES from Internet Options in Control Panel.

Thanks Kimsland - for some reason your reply did not get the usual email advice so I did not see it.
This is a summary of the problem to date - I have not applied RIES yet but will try and will let you know.
Summary so far:
On Wednesday this week 5 machines running either XP Pro or Vista Ultimate and IE 7 all suffered the same problem with IE - it refused to open any URL. The error message was not the usual 404 and all of them were still connected to the net as emails and FTPall worked fine. Refreshing the home page in each case either produced http:/// in the address bar or a message saying the address was invalid.
All machines are running Kaspersky Internet Security and in one case an XP machine had been upgraded to SPIII and Vista upgraded to SPI. System restore made no difference.
HiJackthis run on 2 of them found no problems. By coincidence one of the Vista machines will now not talk to any other of the network computers.
I blame the updates but there is the common factor of Kaspersky. I don't think it is malware.
In each case installing FireFox produced a workaround.
Any ideas?

Is this a hard-wired network or wireless?

Hard wired - and everything else works just fine - and now running FireFox and/or Opera without and problems!

Update to bump
Using IE now essential due to on-line banking needs (darn bank app will only run on IE) so need to fix this ridiculous problem -
update -
1 XP machine with SPIII on it will not run IE7 - everything else works fine including the network.
1 x Vista machine with SP1 on it will not run IE7 or see any other network machines. Has access to the router for FireFox, FTP and mail but will not run IE.
(the other two machines fixed or with workarounds using FireFox it is just these two. Google the error and find nothing - the URL address is replaced with http:/// and in the body of the browser a message sayiing address is not valid (you can force this by typing http:/// into the address line). Nothing seems to work except reinstalling the OS and I am not keen on spending those hours doing that if I can help it!
cheers (in advance, you understand!)

You can remove IE7 then repair (reinstall) IE6

The standard cautions apply whenever you are editing the registry.

If you are having problems only with Internet Explorer 6, log on as an Admin and proceed as follows:

[Start] [Run] [Regedit]
Registry Key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active
Setup\InstalledComponents\{89820200-ECBD-11cf-8B85-00AA005B4383}
Modify the Value Data Type as detailed below;

Data Type: DWORD // Value Name: IsInstalled
Setting for Value Data: [Change the Value from 1 to 0]
Exit Registry
Once you have made the appropriate registry changes use the
link below to download and install Internet Explorer 6. The
Internet Explorer 6 download includes Outlook Express 6.
Reboot and test for proper operation.

http://www.microsoft.com/windows/ie/ie6/downloads/default.mspx#ELC

Does this work with both XP and Vista - many thanks for your input on this! Much appreciated!

No just Xp

Here's the XP SP3 main page now, it makes it easier to link to here as well:
http://www.microsoft.com/windows/products/windowsxp/sp3/default.mspx