The AI in Dawn of War is pretty shocking at the best of times and frequently gets stuck at the first tech level spamming small weak units in an attempt to complete a "tech 1 rush". Typically you will find if you can hold off the initial wave the rest of the game will be a piece of cake seeing as you will have vehicles and they will still only have infantry.
My main point of advice would be to use lots of little squads instead of upgrading one squad (i.e. I would rather have 3 different marine squads of 4 men as apposed to 1 big squad of 10). This means you can engage multiple targets at once and if someone gets drawn into melee when you didn't really want them to you have another couple of squads to help clear the trouble).
As for how to hold off this initial wave it depends on which army you are playing as and which you are fighting. For any army I would definitely say make sure you build a command unit as your first or second attacking unit seeing as the extra HP will definaiely give you a bit more thinking time (this is especially true of necrons seeing as the necron lord is the best unit you have at your disposal) ... from here its a matter of who you are fighting and knowing how they will rush you.
Marines / Choas - Will usually play a solid game with decent AI - harass you a little at the start with marine squads but continue to push through tech levels so you will need to keep up with them technologywise. With choas be careful of cultist squads because they have a tendency to engage multiple squads in close combat at once and leave them prone to fire from another unit.
Elder - they will typically throw guardian squad after guardian squad at you and then throw the occasional farseer whenever they have enough spare rec (strategy = get heavy bolters out asap - or whatever your armies equivalent is).
Orks - Similar to elder, will begin by throwing squad after squad at you but with the added danger of a big mek. This guy can be a pain in the ass because of his teleport ability which will get him to you sooner. (Strategy = you will definitely need a command unit to deal with an ork rush). If orks manage to get to get to tech 2 they are likely to start throwing wartraks at you, counter these with vehicles because their rockets will constantly knock your regular troops over.
Imperial Guard - Again similar to elder, will throw lots of little units at you as well as the command squad which can be a pain because its quickly regenerates powerful units. Heavy bolters (or similar) are the way forward again.
Necrons - On big maps these guys will hardly have the chance to rush you because they are so slow, but you will need to be careful of the necron lord seeing as his teleport power recharges very quick and he can get to your base super fast because of it. Engage this guy with a command unit and have everything else you have shot at him. Probably best to keep his body surrounded as well because when they rebuild him it will revive in the same place it was last killed.
Tau - Watch out for the krut rush, these can be nasty especially if they have feral leap. Lots of separate units will help with this one so that all squads are not simultaneously having to engage one squad.
I didn't ever really play that much Soulstorm so I can't really say more about sisters / dark elder.
I have found a Link on Steam that answers my question perfectly:
Own Boxed Copies of DoW? Get Free Steam Keys
With the shutting of Gamespy servers, we have been migrating our
players over to Steam.
However if you purchased box versions of the game and do not own the
game on Steam, you do not need to re-purchase the games.We are
supplying owners of boxed copies with free steam keys so you can
continue to enjoy the DoW franchise without paying again.
You can request your steam keys by filing a ticket through the
following page:
https://support.sega.co.uk/hc/requests/new
Please note that Sega will need to verify your boxed copies, this may
involve them requesting photos of your box or other information. Due
to the volume of requests it may also take a few days for them to
reply - we apologize this and thank the community for it's patience.
We did not want to see DoW end with the closure of the Gamespy
servers, and are happy to see that our players are still quite
passionate!
Here is the original Link to the steam community Forum.
So as I have the boxed versions of the games with the license keys I will be able to request a new installer.
thanks for all of your help on this issue. Its much appreciated!
Best Answer
I'm far from an expert on Dawn of War, but since no one else is answering, here goes...
For a new player, the Space Marines are probably the easiest to learn. They don't have any unique game mechanics to master (like the Necrons' power-but-no-requisition or the Orks' WAAAGH!), and they don't depend as much on micromanaged special abilities and carefully balanced unit mixtures (like the Eldar or Imperial Guard).
Try the following (approximate) order:
RelicWiki has more information.