While I don't have a usb charger for my DS-Lite, as far as i know your cable should be charging it fine.
The DS-Lite charger i have outputs 5.2V and 450mA.
A Standard USB 2.0 port outputs 5V to 5.25V and 100mA up to a maximum of 500mA if needed (this has changed with USB 3.0)
The only problem you may have is that this maximum of 500mA is usually shared across multiple USB ports on your PC, (educated guess-->) usually 2 or 4 ports depending on the chipset.
Try moving the plug to a different port. You could also buy a USB2.0 PCI card and use that, they should be quite cheap now with USB 3.0 taking over.
Or you could buy a USB 3.0 PCI card which supplies 150mA up to a maximum of 900mA.
You could also try a 'powered' USB hub, but in your case it kinda defeats the point, may as well use the charger instead.