Avast! includes, by default, an always-on, resident scanning service. You should never turn this off, as doing so will leave your system much more vulnerable to infection.
Indeed, an on-demand-only scanner may detect and clean malware for you. But, without a resident component active, it can only do so after you've been infected and damage has likely already been done.
If you really feel that you must disable resident protection, Avast! does provide that option. Still, I strongly recommend against it.
It looks to me like a few things may need to be clarified, here.
"Is there a resident protection so I just launch it and it'll protect my PC onward?"
Yes, and Avast! is one of them.
Most antivirus products for Windows - Avast! included - have three basic components:
- On-demand scanning application.
- Resident protection service(s).
- Updater service.
Items 2 and 3 should be (and, by default, are) always running. These are the services that ensure your system is protected around the clock.
"in the 90% of the cases I don't even use it"
This is entirely untrue. Whether you realize it or not, you are using your antivirus software 100% of the time that you are using your computer. This is because of the #2 component listed above. Every time you run an application, open a file, or even browse a web page, Avast! is checking to make sure that everything is clean.
"Or is it possible to make Avast / MSE on-demand somehow?"
It may be, but most antivirus products protect their resident services from being disabled. In the end, I'd recommend against it anyway.
The resident service is a proactive defense - the best kind. It is constantly checking the code you're running on your computer before it actually gets executed, preventing malicious software from ever downloading or operating at all. Take away this functionality, and you're only left with reactive defense - one that operates after the malware has already found a home on your computer. If you rely only on scheduled scans, there's a fair likelihood that you will find yourself with an infection that will not only be impossible to remove (requiring a re-build of the system) by the time you detect it, but will also probably go un-detected for awhile because it has had plenty of time to hijack your antivirus when you weren't watching.
Regardless of which antivirus product you choose, the resident services really are the most essential parts. While regular on-demand scans are still recommended, full-time resident protection practically renders them obsolete. The down-side of this is that there is some additional overhead added to the boot-up process, as well as any file system operations, but the security enhancement gained is well worth it.
That being said, there is an option to disable the "shields" (as they're called in the program) in Avast! Right-click the Avast! tray icon, and hover over "avast! shields control". You will have options to disable for 10 minutes, 1 hour, until next restart, or permanently. Again, I recommend strongly against using any of these.
If performance is an issue for you with Avast! there are other free antivirus scanners that perform similar functions. Microsoft Security Essentials, AVG, and Avira are among the ones I see most recommended, other than Avast!. Perhaps those will fare better on your system. In any case, I cannot emphasize enough how important it is to keep the scanner's services running in the background always.