@ normski-
Regarding your NetWaiting, do you use dial-up to connect to the Internet? If NetWaiting is a program you use, you need to tell Avast to
trust this program. The way you responded to the prompt you got could explain why you are having difficulty connecting now to the Internet because you said "ignore" and "don't ask me again."
If you uninstalled Avast, and uninstalled v.6.0, then you should not have this problem as you said you are now using v.5. Please tell me exactly what version you reinstalled. Also, since you are now over 20 posts, you can also enter your Profile information in your Signature so that I and others can help you better. Thank you.
I agree with Daris on cleaning your machine. Having something like CCleaner Slim version, which many of us use, is a good idea and light on your system
http://www.piriform.com/ccleaner/builds for cleaning your machine and it can also clean your registry (you can make a back up prior to cleaning/fixing any registry errors). I just checked their web site and they are doing an upgrade so the Slim version should be posted within a day or so.
To replace SB, I would suggest MBAM (Malwarebytes)
http://www.malwarebytes.org/, which many of us use here as well. There is a free on-demand and one-time paid resident version. MBAM is compatible with Avast.
Let me know if you have any questions. Thank you.