xsane says"no devices available"with my hp scanjet 2100c scanner.

I believe you need the Plustek backend:

http://www.gjaeger.de/scanner/plustek/

how do I install plustek install download

You can try with the following:

1a) check if your scanner module is loaded already: lsmod | grep scanner
2a) If it is not, load the scanner module: modprobe scanner vendor=0x03F0 product=0x0505
3a) Add to the end of your /etc/sane.d/plustek.conf file the following lines:

[usb] 0x03F0 0x0505
device auto

To check if installation succeed, try the following:

1b) install the sane-utils package (sudo apt-get install sane-utils)
2b) launch in a console the sane-find-scanner program
3b) if the result is nok, check if your scanner appears following a "cat /proc/bus/usb/devices" command

Note that if step 2a) is needed, than you have to modify your boot configuration so that the module is loaded at each boot

I tried the above still wouldn't locate scanner,I had the same response with a primax colorado.in terminal it says sane already latest available.

What is the output of the sane-find-scanner program and of the "cat /proc/bus/usb/devices" command?

Hi,sane-find-scanner says no scsi scanners found and no usb scanner found.cat /proc/bus/usb/devices normally picks up another piece of hardware ie printer,keyboard and something called teac-usb storage device.sorry to be a pain, And thankyou for your help.

No problem, we are here for this.
Apparently the device is not even recognised by the kernel.
Lets first rule out an hardware problem:
can you use the scanner on another computer?
If you have a dual boot PC, can you check that the scanner is working on the other OS?

I'm not able to test the scanner on another computer,not for a couple of days anyway,I don't know whether it is relevant but I have had the same problem with a primax colorado.Thanks.

Can you at least try another USB port?
I would also be interested to see what is reported by udev. Open a console and type "sudo udevmonitor". Tell me what is reported when you switch on and off the device.
You can close the monitor program with a ctrl-C.

Hi,When I run sudo udevmonitor in terminal and with the hp scanner plugged in,I switched it on and off nothing changed,However when I did the same with the primax it recognised this by showing udev (1177426384.791897)add /class/usb_device/ usbdev2.5 (usb_device)this was taken from a long list this was the final line.It showed something similar when I switched it off exept with remove instead of add.Does this mean the hp doesn't work,Can I get the primax to work?I haven't used either so I don't know whether they work at all.

The hp scanner doesn't seem to work.To be sure, check it with another computer and/or OS when you can.
Bad news also concerning the Primax Colorado, according to this list it is most probably not supported:

http://www.sane-project.org/sane-mfgs.html#Z-PRIMAX