A cardioid IS "unidirectional". Indeed, if you look at the ST51 spec
HERE you'll see that it's another cardioid.
No mic is truly unidirectional--the pickup in the off axis directly is very much reduced but not eliminated. There are some things to try;
1. Distance. Sound follows the "inverse square law" which means that every time you double the distance from a sound source, the amount of sound picked up is quartered. Therefore, if the mic is 2 feet from the laptop now, if you can change that to 4 feet you'll get a quarter the fan noise...at 8 feet it would be a sixteenth, and so on.
2. Precise aiming. The null point of the mic is directly behind the capsule with pickup gradually increasing as you move around the mic. If you can point the mic so the laptop is directly behind the capsule, this'll give the least pickup.
3. Mechanical decoupling. If you're using a desk stand, change to a floor stand to avoid vibrations being transferred up the stand.
4. Acoustic treatment of the room. Proper acoustic treatment is best but even if you can't manage that, somthing soft behind you will help since at least some sound from the computer will be reflected off a wall directly behind you and into the sensitive side of the mic. Things like putting a corner rather than flat wall behind you...or a window with really thick curtains can help too.
5. Change mics. if all else fails, a dynamic mic will pick up less ambient noise simply because it's less sensitive. However, this'll be at the expense of clarity on speech (you'll get a warmer, more "AM Radio" sound) and you may need a better interface/mic pre if your existing one doesn't have sufficient gain.
Hope this helps.
