The causes given for your BSODs (oldest to newest):
Rtlh64.sys
AvastSvc.exe (VISTA_DRIVER_FAULT)
explorer.exe (VISTA_DRIVER_FAULT)
AvastSvc.exe (VISTA_DRIVER_FAULT) - Pool_Corruption
svchost.exe (VISTA_DRIVER_FAULT - fileinfo.sys
System (VISTA_DRIVER_FAULT) - NuidFltr.sys
System (VISTA_DRIVER_FAULT) - Pool_Corruption
System (VISTA_DRIVER_FAULT) - HIDCLASS.SYS
I suggest installing the latest Realtek Ethernet driver (download Vista and WinServer 2008 Auto Installation Program (SID:1457676) [assuming you're still running Vista here], and do a Repair install if prompted) to potentially solve that first Realtek sys BSOD, but the rest seem to indicate some hardware issue. I see you are overclocking; verify your temperatures at idle and load, as well as power supply voltages (best to check with a multimeter). Run IntelBurnTest, then if that passes, memtest86+ for an extended period of time (24 hours or more) to rule out CPU/RAM issues. If any errors result, go back to stock speeds and test again. If no issues, then your overclock has an issue somewhere.