How to deal with several cores VS 2008

Thanks for your reply Tim, the weird part it those applications had worked well on Windows 7 64 bits for nearly a year, I thought it come from memory because each time it crashed at about 25% or 50% following the server...

It's true I try to reduce images with the following code (declaration part removed to be readable in English) where I had already added the Dispose calls months ago :

public static void ReduceImagesFiles()
{
if (!Directory.Exists(ReducedImagesDirectory)) Directory.CreateDirectory(ReducedImagesDirectory);

foreach (String SourceImagePath in Directory.GetFiles(sourceDirectory))
{
sb = new StringBuilder(Path.GetFileNameWithoutExtension(SourceImagePath));
sb.Append(Path.GetExtension(SourceImagePath).ToLower());
imageName = sb.ToString();

if (imageName.ToLower().EndsWith(".jpg") || imageName.ToLower().EndsWith(".png") || imageName.ToLower().EndsWith(".gif"))
{
NewHeight = MAX_HEIGHT;
NewWidth = MAX_WIDTH;

DestinationFilePath = Path.Combine(ReducedImagesDirectory, imageName);

System.Drawing.Image FullsizeImage = System.Drawing.Image.FromFile(SourceImagePath);

// Prevent using images internal thumbnail
FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);

if (FullsizeImage.Width <= MAX_WIDTH)
{
NewWidth = FullsizeImage.Width;
}

NewHeight = FullsizeImage.Height * NewWidth / FullsizeImage.Width;
if (NewHeight > MAX_HEIGHT)
{
NewWidth = FullsizeImage.Width * MAX_HEIGHT / FullsizeImage.Height;
NewHeight = MAX_HEIGHT;
}

//throw Exception on this line
System.Drawing.Image NewImage = FullsizeImage.GetThumbnailImage(NewWidth, NewHeight, null, IntPtr.Zero);

// Clear handle to original file so that we can overwrite it if necessary
FullsizeImage.Dispose();

// Save resized picture
NewImage.Save(DestinationFilePath);

NewImage.Dispose();
}
}
}

I had received some hints from the following link too :
http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/92ba3d50-ba6b-4b4b-99ee-19e7fc8b50da