When this is not an outright hardware failure, it is usually a device resource allocation problem. (this advice applies to windows PCs only). Open the control panel and get the System icon open. Look in the device list, and check if there are any devices windows has marked with a red cross or any other type of error symbol. Just because there are not, does NOT mean that there are no conflicts!! This is most important to learn! many devices come with poorly written drivers, which do not accurately reflect the hardware. This can mean that windows "overlaps" the resource use with another device resource map without "realizing" it. Resources are scarce (otherwise they would not be called resources) so usually if windows can make this mistake it will!
The end result can be a range of mysterious symptoms: video card corruption, mouse pointer freezes, speaker clicks, keyboard freezes, disks seek and re-seek, and activation of the cdrom causes disk activity. The introduction of DSL to a system often requires the introduction of a new card, either ethernet or a DSL modem card.. and this is a good chance for uncovering device problems. The simplest solution is to remove all the devices from the PC that you can (other modems, sound cards etc), then reboot windows and confirm the problems have gone away. Then add back or enable (in the case of motherboard devices such as USB ports), until the problem comes back. If you can narrow it down to two conflicting cards or devices, refer to websites for updated drivers to see if that fixes them.