the computer is a dell XPS laptop running Windows Vista business version
I don't get an error message as such - its just reverts to the orginal log on screen with the different user log-in options, both of which work normally, but they do not have admin user rights.
Please try this: turn off your computer. Then turn it on, and right after the black screen with white text or the computer company logo disappears, press & release key F8 repeatedly until a menu appears, then you select Safe Mode with Networking (note: if there is no menu, then try the same steps again, but this time, press key F8 a bit earlier). Then login and click Start->all programs->accessories->system tools->system restore and restore the computer to any date before this issue has occurred.
Next, please go to Safe Mode with Networking again, then click below and click on Download Latest Version, to download and install the Malwarebytes Anti-malware software. Then scan your computer: http://www.filehippo.com/download_malwarebytes_anti_malware/ (Do not buy anything; just use its free feature.)