is a legitimate Windows file that executes/loads .dll
(Dynamic Link Library) modules which too can be legitimate or sometimes malware related.
What kind of problems are you having with RunDLL32?
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and How To Not Get or Give Technical Assistance on Usenet and Web Forums
What OS (Win 2K, XPsp1, XPsp2, Vista) are you using?
What version of your anti-virus are you using?
What issues/symptoms of infection do you have?
What actions have you taken so far?