I think my laptop affected by Trojan downloader virus. If I click start button windows logging off, also my L key is not functioning. But sometimes when I try to type on address bar automatically types L continuously. Example: (LLLLLLLLLLLLL). Please help me to solve.

You obviously picked up some kind of virus for your laptop to act like this. You will have to use an antivirus scan to get rid of it or end up wiping out your memory, reloading programs and starting all over again.

I would suggest you get assistance at one of the following:
CastleCops®
Bleeping Computer - Computer Help and Discussion
MalWare Removal • Malware Removal - Website Home Page.

They will ask you to run diagnostic scans that they will analyze. Armed with that information they will start to give advice and assistance.