Well I restored the PC and there is now no trace of Avast. I attempted to remove NOD32 but was prevented because a file called setup.exe is damaged or missing (its there in Nod's setup folder but when I click on it I get the same message). According to Eset/NOD, this could be caused by there being a version of Norton on the HDD. So, I successfully removed Norton, ran its wipe application and used CCleaner to clean the registry. So, now no Norton but I still get the same error message when I try and delete NOD. I've posted on NOD's forum to see whether they have any ideas. Otherwise, I'll have to remove the programme manually.
I can't reinstall windows because this is a PC I brought back from my work so I don't have the Windows CD, otherwise I could use a CD from another machine, but that ain't quite legal is it ? Oh well, back to the drawing board....