please delete
First what version of avast have you got, the latest is 4.8.1169 and has some other tools in its armoury.
Whilst a boot-time scan is good because windows isn’t running I don’t believe it scans absolutely everything. But with 4.8 there is the anti-rootik scan which also runs with that.
The Thorough scan with Archives selected is the most thorough (oops a pun), with 4.8 that too also includes the rootkit scan (as does the Standard scan).
With the 4.8 Pro version it is possible to create a new task and have the rootkit scan run.
when i scan with thorough it finds a rootkit and ask whether i want to delete it, so i do. Then it suggests to restart the computer and run a boot scan to completely remove it. When the scan finishes 0 problems are found and nothing else happens. When i scan again with thorough it finds the same rootkit and asks to restart again.I also scanned my system with another anti-virus and it found over 20 000 threats. So why is avast not picking any of them up?
I have the latest version of avast home edition, windows OS, 1.88Ghz Acer AMD Sempron 3400+, 1GB RAM
What would have been useful would have been the details of the detection, file name and location, etc. without it we would just be guessing.
I would advise before choosing delete anything you first check it out here, or google the file name, etc. to see if you can find any association to the file being a rootkit, etc.
Which one?
On-line scanning or you have this 2nd antivirus installed in your computer?
a had AVG anti-virus installed before avast but it wouldn’t delete any of the viruses, so i uninstalled it and got avast. I’m now running avast thorough scan again and it’s finding those viruses ;D Boot scan doesn’t though.
Can you say what is the infected file name, where was it found (C:\windows\system32\infected-file-name.xxx)?
What avast! version and virus database are you using? (see About dialog of avast!)
I’m using the latest avast just downloaded and updated yesterday. Anyway, the scan finished and there was a lot of viruses with file names of songs and films that i didn’t have, so i deleted all of them(there was like 20 000 of them :o). There are still 3 decompression bombs that avast wont scan but i’m not worried about them and a Win32:Trojan-gen {VC} virus that i can’t delete or even move to chest. I have started a new topic about it in the viruses and worms section and if you know how to get rid of it make a post there but mainly all my virus problems are gone except that one. Thanx avast 8)
Ok. Decompression bomb is just something that unpacks to an unusually big amount of data even though it’s rather small (i.e. has a high compression ratio, for example). It’s nothing to worry about, you are just informed that avast! will not try to unpack the archive (you may not even know that it’s an archive, but it seems like it is) because it may take VERY long to process.
(quoted from Igor: http://forum.avast.com/index.php?topic=15389.msg131213#msg131213)
I’d suggest to ignore these files.
But you can change values into avast4.ini file to configure how avast should work with these files. Click ‘Settings’ in my signature for more info 
Even at boot time?
boot-time doesn’t find any viruses. Fully scans but at the end says that zero is found
And the thorough scanning find it?
Where? Which is the name and the path of the infected file?