The only problem here is a modified file could be as useful as a deleted file if it doesn’t contain
the original required information. Eg, you could save an empty file with the same name and this would be a modify, but the same as a delete.
You can change permissions on files and folders, but as explained, this may not actually help.
One way would be to enable shadow copy if your using a windows 2003 file server and you have sufficient disk space to accommodate it.
This would then give you online restore points for any files and folders.
The problem with preventing users from deleting files is that Microsoft office won’t work correctly when they go to edit the files as it creates and deletes hidden files in the folder which it uses for scratch space. If you remove the delete permission you should set the CREATOR and OWNER virtual accounts to have delete rights. The down side to doing this is that users will be able to delete files which they create. This combined with the shadow copy will probably make a good solution for you.