my computer is Dell N4010 . its hard disk divide into 4 partitions : OEM ,System Recovery , C(Window 7) and D(Data). few days ago, i changed C partition to active in Disk Management (i click "Mark partition as Active"). now when i open computer, it can not start Windows .what can i do about this problem ?

perhaps get hold of a cd (or memstick) with a partition commander type program on it. Then restart the computer from such CD and use the partition commander to reset the active partition to the windows one.

Otherwise just get a booting type CD (recovery disk) and get it going and then change the active partition.
M

BARTPE is a bootable environment you should use it as it has an tool similar to disk management you can then revert your changes. I think it is called disktool

There are different compilations of bartpe with different tools make sure what you download has what you need. nu2.nu has more info

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