The progression of implementation technologies into the sub-100 nanometer lithographies renew the importance of understanding and protecting against single-event upsets in digital systems. In this work, the effects of transient faults on high performance microprocessors is explored. To perform a thorough exploration, a highly detailed register transfer level model of a deeply pipelined, out-of-order microprocessor was created. Using fault injection, we determined that fewer than 15% of single bit corruptions in processor state result in software visible errors. These failures were analyzed to identify the most vulnerable portions of the processor, which were then protected using simple low-overhead techniques. This resulted in a 75% reduction in failures. Building upon the failure modes seen in the microarchitecture, fault injections into software were performed to investigate the level of masking that the software layer provides. Together, the baseline microarchitectural substrate and software mask more than 9 out of 10 transient faults from affecting correct program execution.