Remove AC, battery and then press and hold the power button for 20 seconds. Release and put the battery and the AC back in. Try again.
If it fails to start the machine needs repair.
You didn't tell any details such as Dell 15000 Windows XP or if windows runs at all. You wrote 'desktop' which is not a good description. There is a lot of malware out there so we keep our backups current and scan with the usual titles like MBAN and be ready to reinstall a damaged OS.
One of my students laptops desktop will come up but I can't get into anything. Just shows the hour glass like it's thinking. Please let me know if anyone has any suggestions.