You need to format it back to NTFS (or FAT32 if that's how it was).
-Right-click My Computer - Manage.
-select Disk Management on the left side.
-Your whole HDD should be listed as Drive 0, showing all the partitions.
-select the old Linux partition
-ACTION - ALL TASKS - FORMAT
Also Note: if your system is still booting up and asking what you want to log into (ie. Windows or Linux), you will have to boot into the Recovery Console and run the command:
To get into the Recovery Console: http://support.microsoft.com/kb/307654#3