Greetings Slick

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MBAM is currently compatible with both 32bit and 64bit systems, in fact, I'm running it right now on Vista x64 myself

. The installer for both versions (32bit and 64bit) are the same so you won't need to pay any extra, the installer simply detects what type of system you're running (32 or 64) and installs the appropriate versions of the drivers. Your license allows you to use it on either type of system regardless of whether it's 32 or 64 as long as each license you purchased is installed on only 1 system at a time.
I hope I've sufficiently answered your questions, if you need anything else please let us know.
Thanks for the question and for using Malwarebytes' Anti-Malware

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