Right click on the avast icon on the system tray, there are several options, Pause Provider (select Standard Shield), Stop Provider (select Standard Shield, use if pause not sufficient) or Stop On-access Protection and Exit (it stops all providers, but doesn't exit completely, if it did your email wouldn't work until you restart).
When you have done what you need to reverse the process, Resume Provider, etc.
Although the download instructions state that (most will still download), I hate the idea of being unprotected for even a short time on the internet.
Is this to download an on-line anti-virus scanner? As it isn't advisable to have two resident scanners running at the same time, the Pause Provider would probably be best.
HTH David