I have Windows XP and it has just recently stopped me progressing through websites by telling me that IE cannot display the page when I click on a link within a website or type in username and passwords for websites I have navigated numerous times in the past. This can be anything from ba or yahoo to sporting life. Any suggestions ?Use System Restore :

start, All programs, Accessories, System Tools, System Restore

Restore to a date when all was well.

or

run sfc /scannow ( system file checker ) INSERT the OS CD when asked for.

start, run [ sfc /scannow ]
http://support.microsoft.com/kb/926431

Run the Network Diagnostics tool in Internet Explorer Windows can help you diagnose and resolve the problem. Use the Network Diagnostics tool that is AVAILABLE on the Internet Explorer error message page to find the problem and see any available workarounds or solutions for the problem.

Follow these steps to use the Network Diagnostics tool:
Start Internet Explorer, and then try to access the Web page that is displaying the error message.
On the Internet Explorer error message page, click Diagnose Connection Problems to run the Network Diagnostics tool. When the tool is finished, it reports one of the following results:
It was unable to find a problem.
It has detected a problem. In this case, the tool will provide guidance about the next steps to take to troubleshoot the problem.
Click IP Address and note the IP Address. You may need it later in this article.
Follow the steps in the Network Diagnostics tool to fix any connection problems.
Close Internet Explorer, start Internet Explorer again, and then try to access the Web page.
If you receive the same error message, GO to the next method.
Method 3: Reset the modem or the router
If the previous method did not resolve the problem, you might have to reset the modem or the router, depending on which device you use to connect to the Internet.

Follow these steps to restart the modem or the router:
Disconnect the cable that connects the COMPUTER to the modem or to the router.
Turn off the modem. If you have a router, turn off the router also.

Note If the modem or the router does not have a power switch, disconnect the power to the device.
Restart the computer.
After your computer has restarted, turn on the modem. If you use a router, turn on the router also.
RECONNECT the cable that connects the computer to the modem or to the router.
Make sure that all network cables are securely connected.
Restart the computer, start Internet Explorer, and then try to access the Web page.

Does Firefox work?