Norton may have indeed given you a false positive when it warned you that a program was malware. But it's just as likely--maybe even more so--that Norton's mistake came when it told you that your hard drive was clean.
And I'm not knocking Norton here. These issues apply to every antivirus program in existence.
If your antivirus program identified something as a Trojan before you ran it, and found no infections afterwards, there's a very good chance that the malware is protecting itself from security software. That's pretty common behavior.
So what can you do?
First, boot into Safe Mode and try scanning from there. To do so, boot your PC, and press F8 repeatedly before the Windows logo appears. Instead of Windows, you should get a simple menu. Select Safe Mode. This might work because the malware may not load into Safe Mode.
On the other hand, it might. So a better solution would be to boot into Safe Mode, and scan from there with a portable malware scanner that you can run off a flash drive. I recommend the Emsisoft Emergency Kit.
Download the Kit on another computer, and unzip it to a flash drive. Run the program and have it update the malware database. Then remove the flash drive (safely, of course), boot your own PC into Safe Mode, insert the flash drive, and launch the program.
One more suggestion: Use a bootable malware scanner that bypasses Windows entirely. There are several such tools, all free. My favorites are the Kaspersky Rescue Disk and the F-Secure Rescue CD. Both can be downloaded as .iso files, which can be easily burned to disc. If the computer you're using to do this has Windows 7, you can simply double-click the .iso file to bring up the Windows Disc Image Burner. Otherwise, you may have to download and install a third-party iso-burning program, such as Active@ ISO Burner.
You can also prepare either of these for booting from a flash drive. To do this with Kaspersky, download and run the Kaspersky Rescue Disk 10 to USB devices program after you've downloaded the .iso file. For F-Secure, you'll have to download and launch the Universal USB Installer.
And next time your security software warns you not to open a file, don't open it. Or, at least, scan the file with another security program first.
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