This is interesting, for normal updates this code suggests that another install is in progress (I not sure if Windows Defender is the same). This error can also occur if a dll is loaded that it needs to access, or if the downloaded file is corrupt.
Some thoughts
- If the update is already downloaded, try to install it in safe mode.
- Try to download the latest WD definitions and install manually, get them from the Microsoft Malware Protection Center
Good Luck
My Computer
Computer Manufacturer/Model Number
Custom Hack job
OS
Windows 7 Ultimate 64 bit
CPU
Intel(R) Core(TM)2 Quad CPU Q8200 @ 2.33GHz
Motherboard
GigaByte EP45-UD3
Memory
8 GB Kingston
Graphics Card(s)
Nvidia GeForce 9600GT
Sound Card
Soundblaster Audigy
Monitor(s) Displays
AOC x2 22"
Hard Drives
2 x Samsung 500GB (RAID)
2 x Samsung 2TB
1 x WD Green 2TB
1 x Seagate 1TB
1 x Samsung 500GB
PSU
Thermaltake QFan 750W
Case
CM Scout Gaming
Cooling
Coolermaster V10
Keyboard
Microsoft Wireless Keyboard 1000
Mouse
Microsoft Wireless Optical 2000
Internet Speed
320
Other Info
Winfast PVR2000CP
LG DVD-RAM DVD/RW/DL Super Multi
Linksys WMP300Nx2 Wireless Adapter