Hi, I had the same problem using Win32 Disk Imager (error 5) trying to write the wheezy image to an 8GB SD card. Deleting the partition did fix the problem for me, but I think there may be a simpler way, or at least it worked for me. It turns out that on my Windows 7 machine there was a program running in the background called UMonit.exe. I noticed that if I went into Task Manager and killed it (End Process), suddenly I could read or write another SD card without having to delete a partition first. I also notice that other stuff has started working better: I can now "Safely Remove" a USB drive or SD card, where before it always gave me an error.
I permanently fixed this problem by running the Windows built-in utility msconfig, prevented UMonit.exe from loading during startup, and rebooted. Now everything works great. The good thing is, even if you find you need UMonit.exe later (I still have no idea what it is or why anyone would want it), this procedure is reversible: you can always turn this back on and it will start loading again. Here are instructions for those who want to try this:
1. Click Start button.
2. Type MSCONFIG and press Enter.
3. You will see a window that says System Configuration.
4. Click the Startup tab (the fourth tab from the left).
5. Look for something that says "ChangeIcon Application". When you hover the mouse over the Command column for this item, you will see C:\Windows\SysWOW64\UMonit.exe.
6. Uncheck the box to the left of "ChangeIcon Application".
7. Click the Apply button.
8. Click the OK button.
9. You will be asked whether you want to restart the computer now or exit MSCONFIG without restarting. If you don't have any important programs running or work that needs to be saved, go ahead and restart. Otherwise, exit without restarting, save your stuff, close your programs, then manually restart your computer.
10. After restarting the computer, try running Win32 Disk Imager again (and yes, you will still need to right-click and run as administrator).
I hope this helps somebody avoid having to manually delete partitions. I just performed these steps on a second SD card and it works perfectly now.