Why does avast find stuff in boot-time scan which it didnt find in normal scan?

Umm yeah, not much else to say. It found like 3 trojans after not finding anything in a normal scan just a day or less b4 that. So why's boot-time scan more perceptive?

 

The only possible thing I can think of is when your computer starts Windows/Linux, the trojan in the background stops your computer detecting it, whereas at a boot scan, the trojan hasn't started up, and therefore cannot stop avast finding it. I'm probably wrong, but it's the only possible explanation I can think of.

 

Yes it's possible that the virus could operate in a kind of stealth mode. A boot time scan prevents other services from running, hence the virus is not active at this time either and not able to hide itself.

Thanks for pointing this out

 

A rootkit can alter the results of a scan. Pre-boot scans have the benefit of running before the rest of the system has loaded.

BTW, The boot-time scan only applies to Windows unless you're booting from a bootdisk. If you're multi-booting with Linux, it's extremely probable that only Windows was effected.