Longer fake block chain with "valid" transactions
Bitcoin clients always accept the longest valid chain. On one hand, clients check the received blocks for validity, on the other hand length is defined by another metric than many assume. The length of the blockchain is the sum of the difficulty of all its blocks, not the number of blocks. So, in order to create a fake chain, the attacker would have to actually create a valid chain at the same difficulty. This is either possible for very short chains (double spend attack) or requires a major share of the network's power (51% attack).