12k is a bit tight... You can go for Sempron LE 1200+ with Asus M2A-MX combination (3800/-) or Intel Cel 1.8 GHz with Gigabyte 945 based motherboard (4300/-)... You can go for Sony DVD Writer, 1 GB DDR2 RAM, 80 GB HDD, Basic Cabinet model, and local kb and mouse... These together will come to around 9k (assuming the AMD approach)... If you get monitor 15" from cheaper brands like Zebronics, it will cost another 3.2 k... Then, UPS will set you back by 2k... So, without UPS it will come slightly more than 12k... With UPS, it will come near 14k...
Arun
NOTE: The rates are taken from *www.deltapage.com/ which has the price listings of a Chennai based shop... If you are from other towns, there will be some variations in the rates...