automate PDF merging?

Windows Server 2003.
How do you automatically merge all the pdf documents in a specific folder? I need to scan all the folders in a directory for pdfs and combine all the pdfs in a folder into one pdf still in that folder i.e. D:\folderA\1.pdf, 2.pdf, 3.pdf > D:\folderA\123.pdf. I need to automate this so that all new pdfs will be added to the combined pdf.