well finally this is what worked for me (its a combination of different procedures mentioned)
1. download
http://myfreefilehosting.com/f/440c067a37_0.1MB name this file something like boot sector.dat
2. download HDHacker
3. download from SDM - COMMONFILES.ZIP extract this and look for sdministe.exe
4. if ur laptop cannot read more than 2gb card then get one USB CARD READER
5. put card in card reader and start sdminste.exe. this should now show ur card drive letter
6. click on CREATE PARTITION. then click on 2 PARTITIONS. this will automatically create 1st partition as fat12 -2mb and 2nd partition as fat32 (larger partition). as and when it asks u to remove and reinsert card- do it.
7. insert card. run HDHacker. make sure ur card drive letter is selected (REALLY MAKE SURE OF THAT). select LOGICAL DRIVE. select first sector. click on load sector from file. browse to the file - bootsector.dat
8. click on -write sector on disc. now the card should be bootable
9. download the correct version of CHDK for your camera (zip file)
10. extract files directly to the smaller partition of card. only diskboot.bin is needed in small partition (i had to repeat this step 2 times and with error messages it finally loaded that file)
11. insert card. start sdminte.exe
12. click on SWAP PARTITION. (itll ask u to remove and reinsert card - needless to say u do it like a slave!). now the larger partition becomes the 1st partition
13. unzip the chdk to a folder on ur laptop (wherever u like). copy the unzipped CHDK folder tothis larger partition of card
14. again start sdminste.exe. click on SWAP PARTITION (remove and reinsert whenever .........). now the small partition is 1st partition
15. LOCK CARD . put in camera . READY TO SHOOT!.....
9.