The security issue shouldn't be too much of a problem, but you need to run firewall software which you can configure, such as Zone Alarm.
Routers allocate IP addresses to computers connected to them from a "DHCP pool", a range of IP addresses within the Subnet.
Let's say for example, the router's IP address is 192.168.1.1 and its "pool" from which it allocates addresses is 192.168.1.10 to 192.168.1.30, your guests connecting wirelessly will get an IP address ending between 10 and 30. You can give your own computers fixed addresses outside this range, e.g. from 192.168.1.40 to 192.168.1.50 and configure the firewall in your computers only to allow access to computers with addresses ending between 40 and 50. The "guests" won't be able to access computers in your range.
For wireless access in the apartment, if you can hard wire to it, you could run an ethernet cable from your router to the apartment and attach a "Wireless Access Point" to it. Guests would then connect wirelessly to that separate Wireless Access Point.
Alternatively, you could attach a wireless "Cable/DSL Router" to the ethernet cable, which might be less expensive than a stand-alone Wireless Access Point (which tend to be excessively expensive for what they are).