Lost administration priviledge

Virus heat took over computer and now cannot log in as administrator.
Friendly "welcome"screen doesn't come up, so can't use alt+ctl+deltrick.
Can't change user priviledges since not logged on with admin priviledges.
User name I log on with does not show on User screen.
Computer management tree does not show "Local user groups" either.
Can't use MSDOS to reformat either - need admin priviledge.
How do I break in to get admin priviledge back????
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