assuming u are running winXp.
winXP itself can resize ur partions size but it's not save and most probably u will lose data ur small drive is contatining (win Backup feature doesn't suit us). so, most save and easy way is to download soft disk manager software. I'd recommend Acronis Disk Director Suit. This is quite cheap and reliable soft, able to resize/delete/split/merge partitions.
besides unlike other programs has recovery feature (if u delete anypartition accidently then it's quite easy to restore it usng ADDS).
as another solution u can use winXp backup feature and backup some files from C to D drive, that is just copy them. but i think it's quite boring operation.
so here is link
I have a sony desktop, PCV-RS310 to be technical. On the my computer icon or file i have a c drive and a d drive. my c drive is only 13 gb and is the main drive and my d drive still as about 40 gb left. My computer is saying i have low disk space since the c drive is the main drive but i have plenty of space on the d drive. My question is how do i just have one drive which will be the c drive. The c drive only has about 167 mb left which is nothing so everything time i download it says not enough space and i am tried of trying to switch things to the d drive to make space. i just want one drive. please if anybody knows what to do.

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