Thank you very much for your help.
You're quite welcome, it was my pleasure.
I was wondering if there was any way of telling how long these infections had been present on my machine and if they had come from a download or a website.
Some times, yes, there is a way to tell how long the infection has been on a machine. Some times we can tell by the dates of the infected files, but in this case, with Poweliks, that doesn't work. Poweliks is a registry based malware only. There are no infected files associated with it. It also opens a backdoor and downloads other infections to the machines if left in place long enough. That's where the Zero Access infection came from, Poweliks downloaded it to your machine.
As for whether it came from a download or a website, no real way to know that either. About 2 months ago we saw a massive wave of Poweliks infected machines. Users reported problems after downloading files with P2P programs, and some others reported it just started happening out of the blue. The only way they knew they were infected was their machine started performing poorly.