Which database provider Plesk is using in My Windows server ???

Check the following parameter in regedit.exe:

Windows x86:

HKEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\Config

Windows x64:

HKEY_LOCAL_MACHINE\SOFTWARE\wow6432node\PLESK\PSA Config\Config

Here check the value against   "PLESK_DATABASE_PROVIDER_NAME". This will give you the database provider name( JET, MSSQL or MySql).

:)