Since you insisted, I actually wasted some time on this... :| (Using UPX 3.08w.) Some poking around resulted in this: utorrent file, as is; upx returns this error: "upx: utorrent.exe: NotPackedException: not packed by UPX" After removing Section '.bunndle' from the exe file; upx returns this error: "upx: utorrent.exe: CantUnpackException: file is possibly modified/hacked/protected; take care!" What is section '.bunndle', you ask ? An upx compressed DLL file that 'offer' by default to install a toolbar, a browser plugin that slows down surfing, computer and most likely violates user privacy by monitoring user activity aka bloatware (possibly spyware). Many exe compressors are not design to handle file modifications after exe have been compressed. That utorrent works at all is pure luck. The standalone upx.exe file, used to unpack, verifies the file integrity before unpacking (using checksum) while the internal upx decompressor routine does not. (I haven't check upx's code, just a guess) In other words, security vulnerability, a possibility of inserting ie a virus without upx decompressor noticing. Upx really should fix that ie by adding an option to add file integrity check. No, a microsoft certificate isn't security enough. It's crap. A PR stunt by microsoft to get their greedy fingers on more money for nearly nothing and getting more evil control like apple. You knew this and still proceeded ?! Did you even check the upx code so it supported this ? If you really insist to continue with bloatware spreading, causing your users pain, than add that dll file to exe file before compressing the exe file. That is the correct way of doing it. Not to sound cranky or rude but i have done your work for you. I expect at least a small 'thank you' for my wasted time. Unfortunately, anything that changes the exe after it have been compiled can obscure the virus scan. Both exe compressors and protectors are infamous of creating bugs. I for one thinks; stability and data integrity wins over exe file size.