Lo so che la soluzione non è definitva... ma era solo per farti vedere che è un problema nonostante tutto abbastanza comune...
Nel tuo caso però mi sembra strano che una webcam e un modem possano superare le specifiche di alimentazione...
Prova a vedere da Gestione Perifierche, andando su "Hub principale USB" e cliccando la scheda "Alimentazione", ti dice quanto possono fornire in totale le tue porte (è scritto sopra, sul grigio) e quanto stanno consumando le periferiche attaccate in quel momento (nel riquadro bianco). Ogni HUB USB riportato su quella scheramata è composto da due porte USB fisiche...
Per curiosità che MoBo hai? provo a vedere se esiste qualche difetto del genere in giro per internet...
Sempre per completezza riporto un estratto di un manuale (dal sito della ABIT) in cui viene descritto il problema...
ABIT ha scritto:Question: I cannot get USB to work. What can I do?
Answer:
1) Check that USB is enabled in your bios. Consult your motherboard manual to see how to do this.
2) Windows 95, first edititon and Windows NT does not support USB. All other operating systems have their own USB driver built in.
3) Check your motherboard manufacturer's web site to see if they have a BIOS that provides more power to the USB or fixes any known issues.
4) Make sure that you do not have a device or many devices that require more power than your USB port can supply. A standard onboard USB has 450 to 500 mA of power (this is the USB standard). Examples of power used by USB devices:
Microsoft USB mouse - 100mA
Saitek Cyborg 3D joystick - 20mA
Kodak DC4800 camera - 2mA
USB network card - 180mA
Cannon Scanner N65OU uses 500mA of power just plugged in and up to 800mA when scanning
If all these USB devices are used at the one time they would need 1102mA of power but the total provided by the motherboard USB is only 500mA. The only way to use many USB devices at once is to get a Powered USB hub. A powered USB hub uses 0mA and has up to 5000mA available. If you want to or need to use many USB devices at a time, you have to get a powered hub. Some devices like scanners, even on their own need a powered hub. They can run off the motherboard, but it is not recommended if they use over 400mA. Some devices can also be plugged directly into power and use self-power mode rather than using the bus-power provided by the motherboard. You coudl check to see if this is an option for any devices you use.
To find out how much power your devices are using - go into System Properties from Control Panel. Go into Device Manager under Hardware and under the tab for Universal Serial Bus Controllers. Go into USB Root hub and look under power. It will list the mA for devices plugged in. There will be a USB root hub for every two ports you have available on your board. This will vary between 2 and 6.
5) Other things that will effect power to the USB are 1) BIOS settings made by motherboard manufacturers 2) Power Supply - an insufficient power supply will reduce power to the USB 3) CPU speed - the higher the power consumption of the CPU, the less power will be supplied to the USB and 4) the more peripherals you have in the PC will reduce the power to the USB
So di non averti dato una soluzione, ma non so come poter riprodurre il problema... proverei comunque, come già consigliato, un HUB USB esterno alimentato... ma prima di comprarlo vedi se puoi provare...