NOD32 antivirus version 7 can be set to do a virus scan when the PC is idle (and not on battery power), the previous versions were typically set to scan at a specific time such as 2am on Sunday night (or "on next start-up").

Inevitably "on next start-up" is when you need to use the PC, that's why your starting it up right? So this is why you may get the impression that its frustratingly slow "every time you come to use it" this is because its starting up, doing an antivirus update, windows update, app updates and also an antivirus scan all at the same time. No matter how powerful your PC that's a lot of hard drive activity all at once.