NTPASSWD (find with google) can reset that password for free.
For those that need support you go to http://www.lostpassword.com
XP does not want an antivirus and block you so I'm going to write it's probably malware.
Before you do much you should consider that malware can do great damage and we'll want the Dell restore CDs handy. With that out of the way and the password reset you can take Grif's advice at 7726-6132_102-5098912.html?tag=posts;msg5099421
Bob
My friend has just died and left me his Dimension E520 Dell Computer, it hasn't got much in the way of programs, so I have deleted all I don't need. I have installed a TP-link and got it onWireless. But I can't get through to the internet, everytime I try to get a program like Google or Yahoo it returns back a box as shown in the attachment it's saying that it wants administrator to sign a Password which I don't have. The computer is saying it wants a antiviris program but it wont allow me to get through this block. I have considered getting rid of every thing but then I havn't got a XP disk and no one has found it.and I'm not sure I know how to do that . I can't see anyway to send an attachment so here i t is, TO HELP PROTECT YOUR COMUTER WINDOWS HAS CLOSED IT DOWN I get the same answer which ever program it is. Cananyonehelpplease

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