I'm using a powerful PC which is dual-booted Widows XP with Windows7. I no longer can use XP since some upgrades were carried out and I wouldn't want to now anyway, so I am wondering if I can just install Linux either Mint or Ubuntu over the top of the old windows XP? Is this likely to go smoothly or cause me problems?

you never know if you dont try... installing linux is pretty easy. i recommend running a live-usb of your favorite distro to ensure your wireless card, graphics adapter, printer, ... work as expected then run the install-to-harddrive or whatever icon on the desktop.

There should be no major problem with that plan. Some things to be careful of:

The Linux system should be installed creating new partitions in unpartitioned space, not reusing the XP partition(s). That means you need to first delete the XP partition(s). I assume before that you copied any files you needed from the XP partition(s) to elsewhere.

If you use a Linux partitioning tool (typically available before install after booting the Linux install CD) to delete the XP partition(s), they will have very different ID's and appearance than you are used to seeing in Windows. Make sure you understand what you see well enough to delete the right partition(s). Alternately, before booting the Linux installer, use the partition manager in Windows 7 to delete the XP partition(s) and not create any new partitions. (A common beginner mistake is to use the Windows disk manager to create empty partitions for a Linux install. Never do that. You must either use a Linux partitioning tool to create Linux partitions or let the Linux installer do that for you.)

Once there is unpartitioned space make sure you understand and select the choice in the Linux installer to install into that unpartitioned space and not use the whole drive.

Currently you are using the Windows 7 multi boot capability to choose between Windows 7 and XP. If you accept default choices when installing Linux, it will set up a Linux package called Grub as the multi boot capability to choose between Linux and Windows 7. If for some reason you prefer to continue to use the Windows 7 multi boot capability to choose between W7 and Linux, that is possible but would require more research to get the details right.

You shouldn't have any problems except that your windows boot entry still has 2 entries so you will have to remove the winsxp entry or when you boot to wins 7 there may be the option to boot a non-existant winsxp.

A number of issues can arise. If you are worried then just use a virtual machine under windows 7. You could even use the space in the xp area for the virtual hard drive(s).

I seem to remember reading that the Windows partition has to be first. Would it make a difference if he had XP and Windows 7 in that order and then tried to install linux where the XP was?

To original poster: You should list your pc specs for best help on what linux distro (and version) would be realistic to work well on your machine (you mentioned ubuntu and mint, but those each have a few versions that you will need to pick from and some of that decision depends on how old your pc is)

I think you misunderstood something you read.

Should be no problem.

If one installed Windows 7 then the newer loader bcd would be in place.

It should be pretty easy to delete the xp partition and install grub but on some disk's you may have to force /boot to bottom of drive. Can always move and change sizes of partitions with apps. Gparted and maybe Windows can do.

That depends - you might actually find the bcd in the XP partition as it has the boot flag, and the Windoze loader always uses that.
In the past subsequent Windoze installs always updated the prior (already installed) systems boot code.

Best solution is to boot a Linux liveCD and go here, do as it says, and post the RESULTS.txt so we have saome real info on the boot enironment.

i remember having problems with windows being mad that dreve c: wasnt = to /dev/sda so it refused to boot (unless things changed with windows-vista/windows-7/windows-8/... i think windows has to be the first partition on the first hard drive).

also windows will erase grub from the mbr if installed 2nd (most linux distro installers are courteous enough to ask where to put the boot loader). its usually easier to install windows first then install linux.