spotify annoys anti-virus software

it looks like there's a general problem with v3.21 and anti-virus software. Since I installed v3.21 it jacks CPU to 100% (or near) and can take 5 minutes to launch. Only it's not spotify that pegs CPU; it's msmpeng.exe the MS Security Essentials AV program. I can get around it by preventing MSSE from scanning the spotify folder, but that's not a long-term solution. Doing a bit of digging, it appears that a number of people are having problems with a number of different AV applications. So it would appear that there's something about this version of spotify that's upsetting AV applications. What is it?