In Windows Vista Ultimate 32-bit edtion IETester will crash when you attempt to close it if you have a IE6 and/or IE5.5 tab open. This is the error Vista gives me if it helps.

Problem signature:
  Problem Event Name:    BEX
  Application Name:    IETester.exe
  Application Version:    0.2.2.0
  Application Timestamp:    483be972
  Fault Module Name:    urlmon.dll_unloaded
  Fault Module Version:    0.0.0.0
  Fault Module Timestamp:    486468d2
  Exception Offset:    76eefa10
  Exception Code:    c0000005
  Exception Data:    00000008
  OS Version:    6.0.6001.2.1.0.256.1
  Locale ID:    1033
  Additional Information 1:    b637
  Additional Information 2:    cdf75206026ae7e2a6a2625eb045b8f7
  Additional Information 3:    4623
  Additional Information 4:    be24b0993baa00fb2355fb304c54e7fe