The minimum system requirements for Microsoft Flight Simulator X are:
Operating System: Windows 7, Windows Vista, or Windows XP SP2
Computer Processor: 1.0 GHz
Memory: 256 MB of RAM for Windows XP SP 2
512 MB of RAM for WIndows Vista and Windows 7
Hard disk space: 14 GB available hard disk space
Video Card: 32 MB DirectX 9-compatible video card
If you do not have enough space on your current hard drive you may consider adding another one and install the game on that one.