I'll start by pointing out you mentioned Windows XP. XP is hitting security update end of life next April, which basically means Microsoft is done with support and they're throwing the OS to the vipers and wolves. The age of the systems, the potential inability to be quite up to scratch with Win7/8 support (though it likely wouldn't be by much), the cost of newer Win7/8 licenses, and the shockingly cheap cost of Win7 friendly and pre-loaded desktops tosses some interesting wrenches into the works. Linux distros like
Ubuntu might be of interest and use as well, but then you mentioned software you're attached to in the Windows ecosystem.
The answers to your questions ultimately boil down to how much you value you personal time versus cost, how much of that software you can even use on either the Windows 7 or Windows 8 platform (or possibly even WINE emulation under Linux), if there's F/OSS alternatives that you can get for free that replicates that functionality (for example LibreOffice for MS Office 2003) and if they're cross platform or Windows (or Linux) exclusive. Here's some research tools to help on that front to see what your options are:
http://alternativeto.net/http://appdb.winehq.org/Whether you're upgrading hardware or installing a new OS though, if there's old paid software you want to keep using, you're going to need the installers and keys. There's hope for recovering the keys without the physical media on the existing system, and there's ways to get a hold of the media itself without contacting your old support guy, but for the sake of safety and security regarding uncompromised code and installers and the whole DMCA/copyright sticky wicket... I'll only address the software to recover the keys:
http://www.magicaljellybean.com/You can probably read between the lines on the other subject. Your best bet is likely to start looking for F/OSS alternatives (for either platform) or factor the cost of legally upgrading if the alternatives won't cut it.
The fact of the matter is, you can get several nice refurbished Dell, HP and Lenovo (business class) desktops pre-loaded with Win7 and a year warranty
for under $200 a pop through Newegg... versus just the cost of the Windows OS itself (plus possible necessary hardware upgrades if even available) if you're sticking with current hardware.
Finally, as a means to perhaps better help make your minds up on direction and current state of hardware starting with a simple visual inspection:
http://www.badcaps.net/pages.php?vid=5If the visual inspection comes back okay, maybe download, burn and learn to use the following utility which is a useful little tool for identifying hardware issues with hard drives, processors and memory (the three most detectably common failure points in a system - if the mobo is having problems, it will likely show up in one of the extended memory and/or CPU tests):
http://www.ultimatebootcd.com/Last bit of advice, sometimes hardware failure starts in the power supply. It can be dangerous if you don't know your way around electronics to tear down, inspect and test a PSU... so if you're suspecting it, maybe give this troubleshooting flowchart a try instead to see if you can isolate it:
http://www.fonerbooks.com/power.htmGiven the age, if it is the PSU, you'll have difficulty finding a proper ATX replacement that has the proper voltage rails for an ATX 1.x spec system... though being a Dell, you'll probably want to hunt for a 1:1 matching part as opposed to going generic anyway. Personally? Given the age, I'm suspecting bad caps either in the PSU or on the mobo... but I could be wrong.
Anyway, that should help you make an informed decision on what to do, and help eliminate needing to pay someone else to do it.