u need to partition the HDD 1st.. ur best option is, use a windows installer boot disk.. boot ur PC.. create partitions(min2) for eg: 1 partition for windows approx 20GB, n leave the remaining.. format it, install windows..
u now hav another 20GB RAW space.. now insert the linux boot disk.. start installation.. at a point it will ask u wer to install.. 1)delete all partitions, 2)delete only linux partitions, 3)do not delete partitions, use free space..
u select the 3rd option..(the options may vary with the kind of linux distro)
n continue wid installation, at a point it will ask u "default boot" depending on which OS u want my default select (i.e. windows or linux).. sometimes linux may refer windows as "other".. n now finish the installation.. tht shud be it..
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its better to install windows 1st n den linux.. coz linux has got a better bootloader.. n the process is smooth..