I have no idea what avaliable in your country. With $500, I would recommend AMD.
Windows 7 premium 64bit OEM $110 or use ubuntu linux
Case - $40
PSU - quality 400-500W - $60
DVD-burner - $30
2 x 2GB DDR3 1333/1600 - $80
AMD Athlon IIx2 245 $70
AMD 785G chipset AM3 motherboard $80. (prefered brand Gigabyte/ Asus. Asrock/ MSI if you on budget. Get one with 4 x RAM slot for future upgrade)
Harddisk for what ever size you like (prefered brand Seagate/ WD)
Essential Software - openoffice, 7-zip, Avast! / AVG anti-virus, PrimoPDF, Foxit