Actually, if you go to Local Protection->Scanner->Manual Selection, you can select individual drives to scan. If you only want to scan specific directories, right click on them in windows explorer.
Avira uses avnotify.exe to display a pop-up ad only when it updates virus defs, which should be once a day. If you have XP pro, you can software restriction policies to block this program from ever running, without impacting the definition updates.
I've tested Avast, and it's less efficient than Avira with system resources, while providing (from what I have seen from AVcomparitives) a worse detection rate. My advise is to stick with Avira if you want a free AV, until something better comes along. If I was to buy an AV, it would be Eset Nod32, hands down. Much lighter than anything else I tested while often delivering better protection than any of the bigger brand name products.