Unless someone has the drivers and is willing to share them with you then you can't.. I managed to reformat a tiny pc last month and put XP on it. I was lucky as everything worked without the drivers apart for the 56K modem as they was planning to go Bb with cable the following month they went without internet for the month.
I know Modems are so cheap nowadays if it a simple case of a moderm issue, for one which will be easily usable with available xp drivers, then it may just be wortwhile changing the modem over, and anyway you may find you get a faster one anyway.