1) You can't crack someone's computer using Dropbox. RDP is more likely to be vulnerable,
2) you mention a lot of apps, are you afraid someone will take your data from dropbox and onedrive? that will only happen if your password is used. How a cracker obtains your passwd may vary depending on too many things to mention.
Your safest choice will be to use a proper antivirus like ESET Smart Security. Windows Defender is a joke.
If you are trying to find out how safe & secure you are, you might want to hire a security expert to test your PC /Server