hey,
I just scanned my memory and he found a trojan ( SCkeylog-B ) in the explorer.exe file and I am afraid he will delete the explorer.exe file and I will be unable to boot. what to do 
A lot would depend on the location of the reported infected explorer.exe (since this is in memory you may not get an HDD location), example (C:\windows\system32\infected-filename.xxx) ?
Does a search find more than one explorer.exe?
Check out this and see if you have any of the symptoms for SCKeylog-B http://www.sophos.com/virusinfo/analyses/trojsckeylogb.html
You could also check the offending/suspect file at: Jotti - Multi engine on-line virus scanner if any other scanners here detect them it is less likely to be a false positive. You can’t do this with the file in the chest, you will need to move it out.
Or VirusTotal - Multi engine on-line virus scanner