when I started using ccleaner ??

While several programs of this kind are considered "useful" by lots of people, increasing numbers of more sophisticated advisors are reporting more frequently on cases where using one has caused significant problems by removing things the program "doesn't think are useful" that are actually needed by the individual user.

Computer currently in rehab

Probably a good idea when the obvious fixes don't work.

Can a Trojan travel through an internal network?

A Trojan - or any other form of malware - can travel via any method that allows it access to the machine. Some time back, floppy disks - often with simple games that "someone" played on an office computer - were a prime vector for infections. Now we use thumb drives and flash memories (including the ones in you phone or your camera) and the games that people play "on line" to spread much more complex junk, and once ONE PERSON lets it get on a network of any kind, anyone (or everyone) connected to the network can be infected.

Comments already made in this thread are "mostly plausible," and there's little I could have added. Most of the suggestions, though, are only "possibilities" from among the numerous ways that the symptoms described can occur. I haven't found a clue sufficiently incriminating to offer anything.

Just one of the many, a recent report on the "recently popular" bot called Zeus (from the UK PC Advisor) might be of interest, althugh there's no real reason to suspect this particular one more than numerous others in this case, and the symptoms don't really match any particularly common single kind of malware. (Well-designed malware doesn't visibly affect the computer while you're watching it.)

John