Windows lags/jerks when turned on while internet is on

I did not know which section of the forum to post this on, so i chose windows 7 general. If i turn the computer on while my modem is working, my computer becomes slow. Meaning, for example, when i open a folder it will not open smoothly but like in 2 or 3 frames. same thing when i want to move from screen to screen.

However, if i disconnect the internet it becomes smooth. it stays smooth even if i connect afterwards. what can be the cause of this. do i need to worry about virus/trojan/worm? or about configurations or drivers?

Can someone advise?