Packed:W32/MysticCompressor.gen!A is a generic detection for a packer called "Mystic Compressor", which is commonly used by the ZBot/Zeus
trojan (password stealer programs) and rogue antispyware
programs (such as XPAntivirus).
Malware authors use packers, or file compressor programs, to compress their malware and thwart security programs attempting to scan, debug, emulate or unpack the compressed file as part of their normal security procedures.
About generic detections
Unlike signature or single-file detections, a Generic Detection does not identify a unique or individual malicious program. Instead, a Generic Detection looks for broadly applicable code or behavior characteristics that indicate a file as potentially malicious, so that a single Generic Detection can efficiently identify dozens, or even hundreds of malware.