my windows XP suddenly started lagging, sounds are crazy. It looks the sounds became "slowlier" and mouse movement is laggy too. The whole system bootstraps slowlier. Memtest OK, checkdisks are done.
I'm pretty sure it's going to turn out to be the OS; perhaps a recent update. However troubleshooting the hardware just to get 'that' out of the way it's a bad idea. It's good to keep a handful of boot CDs around for just this sort of thing.
Specifically a handful of Hard Disk Diagnosis tools on the Ultimate Boot CD would come in the handiest right now.
And as for other handy disks to keep around, here is my recommended list:
Hopes these not only prove to be helpful in this situation but many others for you in the future.