Cactus said to plug your computer speakers (1/8 inch plug) into the headphone output (again, likely 1/8 inch) of your Presonus interface. Then use ASIO with it. He said to forget about the reltek or whatever your on-board card is - you can't use it, the Presonus and ASIO since ASIO will only allow you to use one driver at a time, not both the Presonus and reltek. I said to buy an adaptor, but the headphone out will work too.
There are ways to assign your computer sounds - boot up, music, porno on youtube to your Presonus, but it is likely to have some problems since MS uses a bunch of 8 bit samples and gawd knows what format youtube etc. are. Music interfaces are designed to deliver one audio format efficiently in and out of your computer; reltek etc. are designed to output all audio formats but not record any one format efficiently. If the Presonus has problems syncing up w/ SONAR after watching funny cat videos on FB, that is the reason, tho not all systems do. So pick your possible poison.