and i got to know that in a single harddrive(eg 250GB),
Its not good for the harddisk to have different type of partitions
Not true--you can have any mix of partition types that you want.
but im not in a position to backup everything.
There is ALWAYS a way to make backups--external HD, CD, DVD, Online Services, etc.
If your data is valuable, then make backups---regardless of whether you are going to install Linux.
To install Linux and setup dual-boot, the normal steps are:
- resize the windows partition to create at least 20GB of free space.
- install Linux
<<The typical Linux installer will automatically sense that Windows is there and set up the dual-boot>>
Another good precaution is to be sure you have a Windows install CD so that you can re-install the Windows bootloader if necessary.