From your post it seems Hard Drive is being detected intermittently. With slight modification I would recommend the steps in my previous reply again, this time specific to Dell Vostro 1450. Please follow the steps below and check if this fixes the problem:
1. Confirm Hard Drive is recognized in BIOS. Hit F2 on initial startup at Dell Logo and check if the Hard Drive in the "Fixed HDD" field of the "Main" screen. If yes, follow step 4 onwards. If NO, "Load Defaults" by pressing F9 and then F10 to save.
2. If the Hard Drive is still not detected reseat the Hard Drive. For steps on how to reseat the Hard Drive please refer to the Service Manual.
3. If still not detected, Hard Drive needs to be replaced. *
4. Run PSA and confirm no errors on HDD. To run PSA, press and hold the `Fn' key on the keyboard and then press the power button to start the computer. If test fails, do reply with the error code (Note: perform complete tests and make a not any errors that appear).
5. If no errors are found boot to Safe mode and perform system restore. If system restore fails or unable to access safe mode, follow next step.
6. Run Start Up repair (Vista and Win 7 only).
Note: The steps above require you to reseat the components by removing the system cover/taking apart the chassis. Please perform these steps only if you are comfortable else seek a technician's help. Also, refer to the Service Manual for accurate steps and safety instructions.
* In case of expired warranty scenario, I would be able to provide limited support, which will not cover part replacement, software support and onsite technician.
Do reply with results.