I have a few softwares that can only be run in Windows. So I'm thinking about multiboot. But I've a few questions. Firstly, does multibooting effects the performance of LM even a tiny negligible bit in comparison to when it is installed alone? Secondly, what happens if my Windows partition is effected by virus? Any effect on LM's performance? Thirdly, my plan is to create a healthy C: drive for Windows at the beginning of my hard disk and install Windows 7 first, then the root partition of LM, a swap and finally a home partition. Is it okay? Please help me out guys.
Installing windows first is a good rule of thumb when dual booting. Windows tends to take over the boot-loader when it's installed. Installing Linux 2nd will ensure Grub or which ever boot loader you wish to use is in place with little trouble.
It's not likely a virus will transfer to your linux partition from windows. With out 3rd party software windows can't read EXT file systems. A virus can affect other items like the MBR and get in that way, Make sure your windows instillation is updated and has a good anti virus, AVG or Avast! are my suggestions.
Having a dual boot system should not affect performance of either OS providing you have enough disk space.