are their any error listed in event viewer?
any drivers missing, look in device manager?
every time or just at certain times?
does the same thing happen in Safe Mode?
have you run malwarebytes (free)
have you run a scan with your antivirus?
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other helpful info:
make, model and model of pc
xp home, pro, or media center edition
32bit or 64bit
fully updated?
what is your anitvirus and do you have any other scanners installed,?