awg virusHow could be an .exe file become virus after one year?

some one sent me angel .exe small startup program animasion I scanned with awast norton awg trendmicro it was clean. Today I scanned angel.exe with trend micro it fond a virus in the angell .exe .I dont know why antiviruses programs could not find it before?
Several reasons:

1. A virus (as opposed to a Trojan or a worm) reproduces by attaching itself to existing .exe files. The virus may have invaded your machine on another file, then attacked that .exe and is now there also.
2. Trend micro now has a new set of virus-definitions. It may not have recognized that virus until now. It may even be getting a "false positive" -- a false indication of a virus that isn't really there.

Hope that helps.
There are two possibilities.

1. The file was not infected one year ago, but since then a virus on your computer has infected the file.

2. An update to your antivirus software has caused it to start detecting this file as being infected. In this awg viruscase, there are two other possibilities to consider:

2a. The file was always infected by a virus, your antivirus software was just not able to detect it before now.

2b. The file is not infected, but it contains code that is close enough to the pattern that the antivirus software uses to detect some virus that it wrongly thinks the file is infected. This is called a 'false positive'.

If you don't mind losing the animation, you can just let the antivirus software delete it. If you really want to keep it, you should let the antivirus software quarantine it, and then you can contact their support and send them a copy of the file to let them determine if it is a false positive.
It is possible that the virus could have had something similar to an internal clock. What seems to be a seemingly harmless file has a bomb waiting to go off. It is a simple operation to program and is likely that that is the case. However, the virus could have also originally started awg virusoff attacking small things in your registry. This would be a trojan. It starts with insignificant things then spreads. That is also possible. However, if that is the case, you may want to have someone look at it and back up all your important files.

OR, it could just be a small virus. No big. But I wouldn't take my chances.
Oh it can happen. Often times, virus writers will create filenames of existing files just to hide the virus from detection. An antivirus scanner can go one step deeper, checking the contents of the file (the size is one of them) and determine if it's a cloaked virus.
Oh it can happen. Often times, virus writers will create filenames of existing files just to hide the virus from detection. An antivirus scanner can go one step deeper, checking the contents of the file (the size is one of them) and determine if it's a cloaked virus.
Some anti virus programs can catch some viruses while others cant.