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It may not be relevant but where did you get Service Pack 5? There isn't an official Microsoft SP5. If you are sure that where you got it from was a safe place, then don't worry about that.

There are a number of reasons I can think of why you wouldn't be able to sign into eBay.

The obvious on is are you sure the password is correct and Caps Lock isn't on.

Is the site working properly? Try signing in through a different portal, such as ebay.com if you are in the UK and ebay.co.uk if you are in the US.

Has your account been suspended? Check your emails.

Are you sure you are on a real eBay site? Check the address bar carefully - the home page for each country ALWAYS starts ebay. and has the country code following it:
http://www.ebay.com
http://www.ebay.co.uk
http://www.ebay.com.au
http://www.ebay.ca
http://www.ebay.de
No hyphens, underscores or anything else.

Has your account been hijacked? Try using the forgotten password to get your account working again and back in your control.

The last two could have happened if you have received an email that only *seems* to be from eBay but wasn't and followed any links from the email. eBay emails will always! have personal details that eBay could only know because you told them, such as your name. It will never say something like "Dear eBay user" and have nothing personally identifying in it.

Have you used their troubleshooting from the Help menu? Checked to see if your account exists there? You can do a search on member names and check if the one that you think is yours says the member lives where you do (or where you said you did).

Have you tried signing out? I know that might sound stupid but sometimes it seems like you are signed in but it's timed out and the sign-in doesn't respond properly. I don't think I've ever had this problem at eBay but I have had it at other sites, I'm sure.

And of course, an old standby, try clearing the cookies and temporary files from your computer and try again.

For Internet Explorer:
From the menus - Tools --> General tab --> Delete Cookies --> Okay.
From the menus - Tools --> General tab --> Delete Files --> Okay.

For Firefox:
From the menus - Tools --> Options --> Advanced --> Network --> Clear Now.

That will clear the "cache" without clearing your History, if you still want that.

And if all else fails, sometimes restarting your computer will solve strange problems.