Welcome to the site. Dropbox have managed to address many of their users security concerns of late, however if you want better security please consider SpiderOak.

For a cloud based storage device, I recommend UbunutOne - but you really do need to get your network secure, having a router broadcast it's signal with no password on it is asking for trouble, packet sniffing is so easy to do on an unsecure network.

Anyone sat outside your home running a packet sniffing program like Wireshark/Shark For Root or DroidSheep/FireSheep can easily just connect to your network, run the program, see what your packets are doing, read them and access ALL your accounts, Facebook, your bank account, anything else they might find interesting.

But with regards to online storage, doesn't matter which one you use, without a secure network, your gonna get hacked no questions asked, that's a guarantee.

I'm no expert in terms of PC security but if you have Norton installed and up to date, that should protect your external drives/devices too.

"Norton" + "protect" in the SAME sentence? I must be dreaming.

Well.......

and no, Norton wont do jack for your external drives, especially not if the drives get infected with something like Rammnit/Sality or TDL4.

Do you keep a quality Firewall on your system? Which one? Have you opened any ports on the Firewall you use? Do you stay up to date with Operating System updates?

Yes it's possible but less likely if you're using https websites. A good firewall should stop the majority of intrusion attempts but I honestly won't tell you that you are risk-free when using an un-secured wifi network.

Yes. A determined individual will get onto your laptop, however a determined individual will do the same with a seemingly "secured" wifi network.

YES, VERY EASILY.

Capslock power go!!

The answer that you do not wish to hear is in Post #12