Hi Andy,
Thanks for your help. The problem seems to be fixed by doing the following:
- Restore backup
- Change all relevant passwords
- Fetch and crawl all affected pages
- Get malicious URL's on our domain in Google's index removed from Google's index
See more affected websites (make sure to load the page in Google's cache): https://www.google.nl/search?q="viagra+kopen"&safe=off&biw=1680&bih=881&tbs=qdr:w&ei=AbSgV_uvM8j6aeaHkKAH&start=0&sa=N
Thanks anyways!