<from previous>
9) When its finished, selected the Disk Cleanup tab again and click OK. Again, agree when prompted.
10) Now select the Tools tab and defragment the hard disk. This may take ages, maybe even hours!
11) Once its finished, install AVG - you're still disconnected from the Internet at this point.
12) Ignore the nags from the AVG installer telling you it can't connect to the Internet. At this point, all we're interested in is getting the thing installed - we'll let it update its virus database etc in a while.
13) Once AVG is installed, do a full system scan - it will probably prompt you to do this anyway. If it finds any viruses or whatever, allow it to do what it thinks it needs to do - it will almost certainly know better than you (no disrespect...)
14) Once that has completed, connect back up to the Internet and reboot straight away. When you're back up and running, let Windows update itself as required. Also, go into AVG and click Tools, Update.