Ahh, well that is a start, but if you didn't 'protect' it with the proper protection (as I alluded to) and went back to all your 'favorite' places you could again reinfect the computer (currently it takes only 10 minutes once a PC is connected to the internet to be infected).
But still I would look at the latter part. Testing should be that single PC connected (wired) to the modem from your ISP, if it is a common modem/router, then you would need to turn off Wifi and ONLY connect your PC and see if on the ISP provided device you still have a issue. if so then tell them exactly that is how your setup and your getting XY speeds. They can't deny then it is a problem on their end.
IF the speeds are 'back to normal' then as I said, more devices more people sharing, less speed for everyone.
And when they say 25Mbps, that isn't FOR EACH SINGLE DEVICE, that is for your ENTIRE HOUSE (no matter if you have ONE and ONLY ONE computer, or have 392 smart phones connecting).