it's extremely unusual to use screened CAT5. Screened cable is normally only used in industrial locations where very high levels of electrical interference are anticipated. In my earlier IT days I must have installed thousands and thousands of metres of network cabling in both commercial and domestic properties, and never once did I use screened cable. So far I haven't had a single report of any mains interference problems.
The important things to remember with CAT 5 are:-
1. Keep runs at least 50mm from mains cables. This rule is not because of a risk of interference, but for electrical safety - to protect the CAT5 from risk of a mains contact.
2. Do NOT clip the cables to joists. Use nylon cable ties and pin those to the joists - or devise your own method. The really professional way to run the cables is to use plastic conduits, but under no circumstances must a network cable be in the same conduit as a mains cable - again, this isn't because of an interference risk, it's an IEE safety regulation.
3. Leave plenty of slack - don't pull cable tight - and if any kinks occur as the cable comes off the reel unkink them carefully, rather than trying to pull them out.
4. When you crimp plugs to the cables try not to untwist the conductors too much - around 13 mm is just right.
Finally, my advice is that while you're doing this it makes good sense to run more cables than you actually need. Once the floorboards are down it's no fun to discover that one of your cable lengths has a fault.