Evaluating architectural impact of applications with a significant operating system interaction calls for integrating detailed microarchitectural user-level simulation with system-level simulation tools. This paper reports on our experience in integrating Simics - a system-simulation tool - with Wattch - a microarchitectural performance and power modeling user-level simulation tool built on top of SimpleScalar We first present the technical challenges we had to resolve in designing SimWattch - the integrated tool. We then use it to identify the type of errors a user-level simulator typically does when predicting performance and power consumption while omitting operating system activity. This case study is based on SPEC95, and SPEC JVM98 applications and TPC-B. We find that if operating system effects are omitted, performance is usually overestimated while energy used is underestimated. However a surprising result is that IPC, power and resource occupancy predictions from a user-level simulator often follow the trends of predictions from simulations factoring in operating system effects.