I am trying to access my Win XP in Dual boot after a few months lack of action.
Can select Win XP in boot menu, the program loads but I cannot access it as there is no connect with USB mouse or keyboard.
I tried un/replug both but no action.
I could repair using the install DVD, but am scared witless I'll lose all my Linux (extensive) stuff. Will that action screw my MBR?
Would someone please direct me?

Are you booting with the Grub bootloader? I'm not sure what you mean by the above? Can you actually boot into xp but are then unable to do anything as the mouse/keyboard are not functional? If you have an xp installation DVD, you should be able to repair the bootloader which would put windows code in the mbr. You would not then be able to boot Linux unless you created and entry for it in the xp boot.ini file or reinstalled Grub. This option should not damage your Linux installation?? If you select a recovery option to factory state, well that's a different story.

More details on exactly what your situation is and what your intentions are would help.

Grub 1.99 (Grub 2) should automatically find Windows. If it does, all you need is three different partitions: a boot partition for Windows, a boot partition for Linux, and a data partition for Linux. (Data meaning filesystem, etc.). The data for windows is on the same partition as the boot. Make sure that both boot partitions are set as bootable. Then, you can use Grub 2 to boot either a kernel image or a windows partition.

Hiren's bootcd has miniwinsxp try it. You may just need to reinstall drivers.

XP's when booted to the CD and the proper "repair" should not affect the boot loader.

Lots of suggestions to try, but conflicting answers, but unsure regarding "Hiren's bootcd". Can I ask for more information, please?

seems to be a windows driver issue. i am assuming from the description that the mouse and keyboard work in linux and you are able to use the keyboard at boot to select that you wish to boot into windows (i have a wireless keyboard that wouldnt work because the battery needed to be changed but that doesnt seem to be the case with you).

i think you are rite to be cautious with the windows install cd because windows has a tendancy not to play nicely with other os's on the same disk. i would backup important data to a usb storage medium (fone, camera, usb, cd, sd, ...) and run the win-xp boot disk and hope for the best. i think at worse you would need to reinstall grub to the mbr.

do you have just 1 harddrive ?

Therein lies the problem. You were asked for more specific information after your initial post and didn't provide any. It seems from your first post that you are able to boot xp but that your mouse and keyboard don't work. Is that the problem? If not, be more specific as to what the exact problem is.

http://www.hirensbootcd.org/

That is exactly the problem, as stated in the original post.
So where to from here? If I reboot into repair Win XP, then I'll screw up the MBR and lose Linux. Can't afford that.
I obviously need to repair the USB drivers, but have no idea where to start.
Please help.

If you can boot xp from Grub? but can't do anything when you have booted xp because your usb mouse/keyboard don't work, I'm not sure what putting windows code in the mbr will do to help your situation? I'm not familiar with repair options for xp as I've never really used it. I understand there is an option to restore to factory state which in your situation would be a bad move as that would overwrite your Linux. Maybe you have other options in the repair menu. I'm not familiar with them so maybe someone else will come along who is familiar. Have you tried usb/ps2 converter for the mouse/keyboard?

"If I reboot into repair Win XP, then I'll screw up the MBR "

You must explain this. A Windows repair is not going to screw up the mbr. Loading the recovery console and a fixmbr may but the repair you want is after booting the CD to it's repair choice. This is not the link I wanted but it is there. http://www.michaelstevenstech.com/XPrepairinstall.htm

Perhaps a silly question, but have you tried just waiting?

As you've deduced, the problem has to do with USB drivers. For some reason (new hardware?) Windows XP has decided that the existing driver settings were invalid, and as a result none of your USB devices work.

Unless the USB drivers are missing from XP altogether (which is not very likely, unless all your ports are USB3), XP should detect and install the drivers automatically. This process may take quite some time to finish. In one case, I had to wait for almost 20 minutes before the keyboard would respond.

Best advice to date. I tried booting again, twice. The second time it ran a check disc, after a second re-boot all worked correctly.
Thanks for all the advice.