Well I can only offer generic guidance as I've no details of your system.
It sounds like you are using IE7. You could try and make sure that the security level on your browser isn't set too high. Medium should be enough. If you have tweaked it, you will need to tweak it some more. Sometimes, deleting all cookies from 'pprune.org', quitting your browser and then restarting your browser can help.
Also, have you tried logging on using a different browser? FireFox is one of many good alternatives and they are all free. We firmly believe you should always have a back-up browser because of the problems that IE7 can cause.We also know that some people have reverted back to IE6 but sometimes that isn't as simple as it seems.
Don't forget there's a specific computer and internet forum here for advice.
Rob