It seems that people using Symantec’s Norton and Corporate edition antivirus scanner products have reported very strange Trojan horse warnings about HL2 games in Steam. This report seems to be false and we do not know why Symantec is reporting this as of yet.
Blackhawk from the Steam forums tells us how to fix this:
If you are running Norton Internet Security or any other Norton product and you cannot play CS:S due to Norton thinking it is a Trojan. Here is how to fix it:
1. Open Norton Internet Security.
2. Click Internet security.
3. Click Settings.
4. Click Auto-Protect, then click configure.
5. Another window will open, to the left of the screen near the bottom click “Scan Exclusions”
6. Now where it says “Which disk, folders, or files to exclude from Auto-Protect scanning.
7. Click “New”.
8. Add this C:\Documents and Settings\YOURPCNAME\Local Settings\Temp\
9. Click Ok, once that has added the temp folder click Ok to close the window.
10. Now try running CS:S or the game you had this problem with.
If it is blocked again by Norton all you need to do is:
1. Open up Norton Internet Securtiy again, and click Reports & Statistics.
2.Then “View Activity Log”, then click “Security Risks”, see what Norton is blocking, it should say Trojan Horse and then see what the file/folder is.
3. Then just simly add that to the list that Auto-Protect doesnt scan.
Update: This seems to be fixed in the newest virus definition update.
Please update your virus definitions as soon as possible and the problems should be fixed.
Security Response is aware of a False Positive in Valve Software’s “Steam” for Counterstrike. Rapid Release virus definitions sequence: 70637 will be available within an hour to resolve the issue. A second Certified LiveUpdate is planned for release this evening as well.