There is very bad polymorphic file infector that is probably doing that.  Many security forums are seeing folks with this.  It is capable of infecting all the machine's executable files (.exe) and screensaver files (.scr). However, the problem is that the virus has a number of bugs in its code, and as a result, it may misinfect a proportion of executable files and therefore, the files are corrupted beyond repair. As of now, security experts suggest that the best way to fix this is the format and clean install, or destructive recovery if you have an OEM recovery partition as the best way to clean the infection and it is the best and safest way to return the machine to its normal working state.
I know that is not what you wanted to hear, but that is what we've been dealing with lately.