browser download speed suddenly reduced, browsing is normal

hello people, i need help quickly!
jst few a days ago, everything was okay about my internet
i have a 600Kbits per sc connection, cable modem
so my downlaoding speed is around 60-70 kbytes/s
same in utorrent and in browser downlaods,
i use windows 8.1 pro, 64 bits, recently my google chrome was turning slow and firefox too, i dunno if my pc was infected by viruses. i scanned wit havast and bitdefender free versions and found nothing.
and yesterday when i tried to download a pdf file, i dunno why nut downlaod spee dwas horrible, started with 24 and then it was like 3-4 kbytes/s and it even failed later.
i thought it was a connection problem so i restrted my connection, browing was fine everything except downloading seemed okay.
i tried doing the same things with different browsers like firefox or internet explorer but the problem was same.
shockingly when i tried utorrent, it was perfectly normal, it gave the ususal downlaod speed of 6-70 kbytes/s
so it seems the problem was onl in downloading with broswers and no problem with browsing and utorrent.
i hard resetted my pc and reinstalled fresh copy of win 8.1
but again twas the same.
did it 3 times already but no change.
i dunno if it is a virus or something but i am exhaousted, plzz help soon
( before formattin my pc , i backed up my files in an external hdd so it may be possible taht if tehre is a virus involved then it may have came back due to my external hdd)