friends computer has malware / and is slow

here is the sys Info:
Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i5-2300 CPU @ 2.80GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 4
RAM: 6056 Mb
Graphics Card: Intel(R) HD Graphics, -1988 Mb
Hard Drives: C: Total - 1414313 MB, Free - 1313009 MB;
Motherboard: Gateway, DX4860
Antivirus: avast! Antivirus, Updated and Enabled

The computer is slow and I need it cleaned out to make sure there are no malware.