Macs are Anemic for USB power, get an AC Powered USB Hub to cure the problem...
http://www.buy.com/prod/high-speed-usb-2-0-7-port-hub-powered-ac-adapter-blue/q/ sellerid/23962916/loc/101/213271959.html
Though one or more of your USB devices may be defective.
To get the important info about your Mac,you only have 500mA to hand out to start with...
At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto *but not including the Serial#*...
Hardware Overview:
Model Name: eMac
Model Identifier: PowerMac6,4
Processor Name: PowerPC G4 (1.2)
Processor Speed: 1.42 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 2 GB
Bus Speed: 167 MHz
Boot ROM Version: 4.9.2f1
Then in the same place check the USB section...
USB Bus:
Host Controller Location: Built In USB
Host Controller Driver: AppleUSBOHCI
PCI Device ID: 0x0035
PCI Revision ID: 0x0043
PCI Vendor ID: 0x1033
Bus Number: 0x1b
Apple Optical USB Mouse:
Product ID: 0x0307
Vendor ID: 0x05ac (Apple Inc.)
Version: 3.40
Speed: Up to 1.5 Mb/sec
Manufacturer: Logitech
Location ID: 0x1b200000
Current Available (mA): 500
Current Required (mA): 100
Hub in Apple Pro Keyboard:
Product ID: 0x1002
Vendor ID: 0x05ac (Apple Inc.)
Version: 1.00
Speed: Up to 12 Mb/sec
Manufacturer: Mitsumi Electric
Location ID: 0x1b100000
Current Available (mA): 500
Current Required (mA): 50
USB2.0 Hub:
Product ID: 0x0608
Vendor ID: 0x05e3 (Genesys Logic, Inc.)
Version: 7.02
Speed: Up to 12 Mb/sec
Location ID: 0x1b130000
Current Available (mA): 100
Current Required (mA): 100
USB2.0 Hub:
Product ID: 0x0608
Vendor ID: 0x05e3 (Genesys Logic, Inc.)
Version: 7.02
Speed: Up to 12 Mb/sec
Location ID: 0x1b134000
Current Available (mA): 500
Current Required (mA): 100
802.11 bg WLAN:
Product ID: 0x1723
Vendor ID: 0x0b05 (ASUSTek Computer Inc.)
Version: 0.01
Speed: Up to 12 Mb/sec
Manufacturer: Ralink
Location ID: 0x1b134100
Current Available (mA): 500
Current Required (mA): 300
USB to ATA/ATAPI Bridge:
Product ID: 0x2338
Vendor ID: 0x152d (JMicron Technology Corp.)
Version: 1.00
Serial Number: 152D203380B6
Speed: Up to 12 Mb/sec
Manufacturer: JMicron
Location ID: 0x1b134300
Current Available (mA): 500
Current Required (mA): 2
Apple Pro Keyboard:
Product ID: 0x0204
Vendor ID: 0x05ac (Apple Inc.)
Version: 1.01
Speed: Up to 12 Mb/sec
Manufacturer: Mitsumi Electric
Location ID: 0x1b110000
Current Available (mA): 250
Current Required (mA): 50