What is the configuration of that 12k PC?
Arya, one advice which you might not wanna follow is to switch to thin clients. Trust me, that will solve lots of problems & hassle for u as you don't know anything about Windows.
The configuration I have mentioned will cost you 18k. To make it more simple here is a configuration customized for your needs.
Intel Pentium Dual Core E2140 = Rs 2,500.
Intel 945GCPE motherboard = Rs 2,750 or Biostar GF7050V-M7 = Rs 2,200 if available.
2X 512 MB DDR2 667 = Rs 950
iBall Baby 306 cabinet will cost you Rs 1,800 with a 400W SMPS included. Do not buy anything other then this cos not only this cabinet small but also enough for the hardware you want to buy. Physical space is always a problem in offices so the smaller the computer, the better. 400W SMPS will be enough for your needs.
Buy second hand 80 GB hard disks. They should suffice for the hard disk space requirement & you can get them easily for Rs 800 each. If you still want to go for new hard disks then go for 80 GB SATA hard disks which should cost about Rs 1,500.
No optical drive is required on the nodes.
Motherboard already has Onboard LAN, Audio & graphics.
Logitech Internet Pro Desktop = Rs 750
Since you will be taking calls then a good pair of headphone + mic is required on each node. Have a look at Microsoft LifeChat series. There LifeChat LX-2000 should suffice or look at Altec Lansing & Labtec internet chat products. Since employees will be on call most of the time so comfort is a big thing here.
For the monitor, just buy a 17" Analog LCD to be used with on board graphics of the above mentioned motherboard. Look for Samsung or LG, they cost about Rs 8k & should be enough. Personal preference is 19" LG 194WS for 9K
For electricity backup, install a unified invertar in the office & connect all computers. That is better then buying individual UPS.
Keep in mind these should run Windows XP SP3 so you might not be able to work on it due to unfamiliarity.
Since these computer will do only one thing 24/7, you just need to configure one PC, make a disk image of the partition & deploy on all the nodes. Obviously all teh computers will be connected to a central server so make sure all the backup & saving of data is through LAN & not on Local storage.
I still say, look for Thin Client options, ask the H.P or IBM office in your locality for the cost info.