The first thing is to try it on another computer. Distortion can be introduced in a number of ways:

(1) At the stylus/ record interface: fluff, groove damage, dirt, jam (once when the Boy was small), damaged stylus, incorrectly installed stylus, incorrect arm balance, mechanical damage to arm etc etc.

(2) In the conversion process from analog to digital. Did you install a new cartridge, with a higher output than the old one? This could saturate the analog to digital converter, though only during loud passages. Is there a level control that could have been inadvertently changed?

(3) In the transmission brocess. USB sound is normally transmitted as Bulk Transfers, which means there is no check for lost data packets. If you are connected via a USB hub with other devices connected, you could be asking too much, especially if it's USB 1 or the device is being treated as USB 2 Full Speed (12Mb/sec rather than USB 2 Hi Speed 480Mb/sec). Have you plugged into a different port this time? Sometimes laptop ports are shared: effectively an internal USB hub.

(4) Within the computer- are many other programs running at the same time, or a lot of hard disk IO going on?

Here's a less- than- helpful Microsoft technical note.