Hi,
According to the minidump, you haven't upgraded or de-install KAV. I find the same version of KAV is installed at your windows. One windows crash is crashed by KAV. Two system crashes are caused by faulty RAM.
Mini121806-01.dmp BugCheck 19, {20, e171b580, e171b580, 80002b0}
Probably caused by : klif.sys ( klif+14d1c ) <-- KAV
Mini121806-02.dmp BugCheck A, {a8fffffc, ff, 0, 804db973}
Probably caused by : ntoskrnl.exe ( nt!SwapContext+4f ) <-- Faulty RAM
Mini121806-03.dmp BugCheck 50, {bad0b158, 0, 80566a37, 2}
Probably caused by : ntoskrnl.exe ( nt!ObpCloseHandleTableEntry+13 ) <-- Faulty RAM