First run the diagnostics via the Dell BIOS and any extended tests. See the instructions on page 410 of my Windows Reinstallation Guide: [URL] Also since the system is used and seems to have software problems I recommend performing a clean install. See A Clean Install of Windows 7 also in the Windows Reinstallation Guide.
I've got a used Dell XPS 15Z that has been buggy since I got it. Had a hard time running Windows Updates. A friend at work solved that one by assigning a letter to the hard drive or something.  My Device Manager window shows 4 "other devices" Three of them are as follows:  PCI bus 5, device 0, function 0  PCI bus 5, device 0, function 3 PCI bus 5, device 0, function 4 The fourth is: on PCI bus How do I find out what these are so I can update them and assign them to their proper places?  
SMART Short Self Test cannot run - and other problems