hey,
my friend's computer is infected with about 55 viruses (trojans, backdoor etc), and they're all located in the system folder. Can't be healed or deleted, as a warning pops up saying that the OS might not function properly without those files. Any suggestions?

Download the following free online virus scanners, set them to auto clean and run them from Safe Mode

Trendmicro

Panda

If you are not running an up-to-date antivirus AVG Antivirus is free and does agood job.

To start on the spyware you might first download and install this preventer Spyware Blaster it will need to be updated right away then once weekly, and it runs in the background disabling most spyware before it can install on your computer.

Next run these two spyware removers Adaware SE and Spybot update them both and run them from safe mode.

Then general clean-up.For all users of the computer go to start>my computer>local disk c:>documents and setting>and delete the contents of these folderes if found:
Temp
Temporary internet files
Cookies

Go to C:\WINDOWS\Temp and C:\Windows\Prefetch and delete the contents of these folders.

Then disable System Restore to purge it then re-enable it.

This probably will not get rid of everything but should get your on your way.