Remote Access- Can I tell if someone has accessed my computers files?

My netgear DG834GT router was recently reset to its default security settings, which are very basic. I believe someone has tried to gain access to my files/ browsing history etc through remote access (they have got hold of the original network key to turn remote access on) using a wifi connection. Is this easy to achieve and does this leave a trace on the router or my computer and if so where/ how can i find it?