This possibly is a false positive identification, since you willingly downloaded the mining software. If you downloaed from a trustworthy source (original guiminer/poclbm threads/sites or the site of one of the big mining pools) you should be safe.

There are trojans with the prupose of sneaking mining software into systems and using that system's GPU and CPU power to mine for the author of the trojan, without the system owner knowing that his system is currently mining. Malwarebytes might have falsely identified the guiminer files as such a trojan.

I am not completely sure about that though. Maybe someone more knowledgeable can chime in.