You need to post some system info. So we can better assist you. My 1st question is what firewall do you have up? If you have Comodo Pro firewall you must uninstall it, this was a major problem some months ago. I encountered it for 2 months until I found the solutions....(complete uninstall).

Roger1955

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XPS 410
Windows Vista Home Premium
Intel Core 2 Duo E6600 Conroe @ 2.4 ghz
19 in FP Display/ IEEE 1394 PCI Express Firewire
3 GB DDR2 / Evga Geforce 7600GT/ 13 and 1 Media Card Reader
2x250 GB WD Sata II HDD'S (Raid 1 Array)
Sony CRX 3105S 48xCombo CD-RW/DVD Sata
TSST Corp TS-H553A 16xDVD+-RW/DL Sata
Logitec QuickCam Pro 9000
McAfee Security Suite, Spywareblaster, Windows Defender, SuperAnti-Spyware

Re-register the Windows Update DLL with the commands below

Click Start, click Run, type cmd, and then click OK.

Type the following commands. Press ENTER after each command.

regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 wuaueng1.dll
regsvr32 wucltui.dll
regsvr32 wups.dll
regsvr32 wups2.dll
regsvr32 wuweb.dll

Attempt to run Windows Update