I don't think you'll find any New Old Stock XP era computers.
You're even less likely to find a New Old Stock Windows 98 computer.
You might find New Old Stock parts, like video or sound cards, but even those are getting rarer.
Don't get a computer with onboard graphics (integrated graphics), or if you do, make sure the motherboard has an AGP or PCI-E slot so you can add a video card.
Don't get a little skinny one. They're not likely to have good video and are more likely to have heat issues.
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The best I saw were these two, both with Windows 98SE and nearly identical except for video card.
http://www.ebay.co.uk/itm/262806701758http://www.ebay.co.uk/itm/262805362041Both have Pentium III 1.4 GHz Tualatin processors, a great choice for a fast Windows 98 computer, and the video cards were high end for their time -- not some dinky low end budget cards.
The GeForce FX5900 card is newer and faster than the GeForce 4 TI 4600.
Both are sold by the same person. Going by the description, the seller supports his work and offers technical support if needed. He has 100% positive feedback.
I'm seeing shipping as US $45.00 (approx. £36.95), but it may be more for a UK destination.
eBay won't let me see what shipping to the UK would be.
The computers aren't really best for games from 2006-2007 like Undercover: Operation Wintersun though.
Whether they'd work with Operation Wintersun depends on what is meant by the DirectX 9.0c requirement. Almost any video card will work with DirectX 9.0c, but that doesn't mean the card is "DirectX 9.0c compliant."
The GeForce FX 5900 is DirectX 9.0b compliant, meaning it supports all DirectX 9.0b features.
The GeForce 4 TI 4600 is only DirectX 8.1 compliant.
Both cards "work with" DirectX 9.0c, but I don't know for certain whether the game requires some DirectX 9.0c feature that they can't support. It's very aggravating that game boxes don't make this clear, and I've gotten caught out when I had a faster video card with more memory than what was listed, but it didn't support some niggling DirectX feature that was introduced in an incremental update, so the game wouldn't start. Does the game actually need a DirectX 9.0c-compliant card or is DirectX 9.0b-compliant OK? Who knows? The FX5900 has a much better chance of working with the game than the GeForce 4 though.
Both computers more than meet requirements for these games
Longest Journey 4CD versions (1999-2000)
Syberia (2002)
Dark Fall (2002)
Post Mortem (2002)
Omega Stone (2003)
Watchmaker (2001)
Schizm (2001)
However if you're interested in many other games from 2006 or later, these computers are probably not the best choice, and you'd be better off getting an XP or 32-bit Vista computer. I didn't see any at the current time that wouldn't need upgrading though.
I don't know what your price range is, but the two Windows 98 computers were a lot more suitable than most of what I saw, and wouldn't require that you add a new video card.
If you want something that costs less or that you can be sure supports newer games from 2006-2007, and are OK with adding a video card, there may be other options.