The boot-time scan is used when a virus can't be dealt with with windows running as it may be in use, or protected by windows because it is in a system folder, etc. It is a special tool, which select when you need it.
Because it is operating outside windows you have limited controls and the 'but none of them work.' comment, I suspect you have a USB keyboard ?
If so you need to ensure that the USB devices are recognised before windows drivers are loaded, there is a setting in the BIOS to do this.
As your system boots keep tapping the Del(ete) key and it will bring up the BIOS screen, sorry I can't say were this setting is your the USB devices/keyboards is as there is no standard layout or naming between manufactures. The problem is how do you do this if the keyboard isn't recognised prior to windows, you may have to plug in an older PS2 connection keyboard and then boot. I despair at System manufactures that sell pre built systems with USB keyboards but haven't set the BIOS to recognise them.
Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate. So not a good habit to get into.
Before doing something you should have an idea what effect it will have, e.g. check the help file, etc. Scans have different sensitivity, locations, what files to scan, etc. so if you choose to scan all local disks at a 'Thorough' sensitivity and you choose to scan 'Archives,' it will take a very long time.
Archive (zip, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned. Thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
I have only ever done a through scan with archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.
