I would run chkdsk to make sure the file system is in tact, although if some windows files have been corrupted, chkdsk alone may not resolve the issue. Since you can get to Windows, the easiest way is to go to start > run, type in 'cmd' (no quotes) and press ok, then type 'chkdsk /r' (again, no quotes). It will tell you that it cannot lock the drive and ask if you'd like to run on restart... choose yes, then manually restart your computer. Please note it will take a while to run, so make sure you're not running on battery.
Another thing that would be worth checking is the HDD integrity... restart the computer, and on the Dell splash screen (where it says hit F2 for setup), hit Ctrl+Alt+D on your keyboard. This will perform a quick test on your hard drive (Dell calls it the 90/90 test, takes about 90 seconds or less, and finds 90% of hardware failures). If the result is anything except "Pass," call Dell for more information (or if you're out of warranty, be aware that your hard drive may be on the way out, but not necessarily unusuable yet).
If your hard drive passes diagnostics and chkdsk /r does not resolve your issues, you may need to reinstall Windows to repair any missing or corrupt files. You could also try start > run, type 'sfc /scannow' and hit ok, but I've personally never had it fix all the issues that arise in situations like this.