Are Jim Raynor and Sarah Kerrigan romantically involved before Wings of Liberty?

It's been ages since I played the game, but...

... When Jim and Sarah first meet, there's a definite bad first impression (she's a telepath; his first unguarded thought when he meets her is about how well she fills out that tight uniform; she calls him on thinking it), but a few missions later, they're on first name basis -- enough that when Mengsk orders Kerrigan to protect a nascent Zerg hive from the Protoss, Raynor protests loudly about the danger, and only acquiesces when Kerrigan tells him to have faith because she trusts Mengsk has a good reason for it.

(He doesn't, unless "I sold you off to the Overmind so I can have my empire unmolested" counts as a good reason)

Mengsk's subsequent betrayal of Kerrigan to the Zerg by ordering all air forces to withdraw when the Zerg start overrunning the base is what enrages Raynor to the point where he decides to quit, and his escape from Mengsk's new empire ends the Terran campaign.

The first - and pretty much only - mention of their relationship during the Zerg campaign is during the mission where the Queen of Blades hatches from her Chrysalis -- apparently, some kind of telepathic emissions were manifesting in Raynor's dreams and called him into the area containing the Chrysalis, where he has a few heartbroken words when he finds out what Sarah turned into and that the Queen of Blades is absolutely not interested in being rescued.

For more references, see here and here on the official Starcraft wiki.

Apparently, they were in love, but no details are given.

Shaken by the ordeal, she and Raynor found comfort in each others' presence, their former antagonism having evaporated. However, as strong as their emotions had become, the war prevented them from acting on them. (Quote from http://starcraft.wikia.com/wiki/Sarah_Kerrigan, which uses as a source the novel "Queen of Blades")

PS: Here's a quote from the book itself:

Kerrigan. His friend. Almost his lover. Certainly the attraction had been there on both sides - they had both felt it. And he had almost acted upon it more than once. But the timing had never been right. That was the way with wars - they got in the way of other things.

Starcraft 1 was told in a very different manner than Starcraft 2, not the first of which is that the player character is an actual character unto themselves, actually playing as Artanis in the first Protoss campaign. (Compare this to Starcraft 2, where the player piggybacks with Jim Raynor)

In part because of this, your information on events is fragmented - You can't know if Jim and Sarah are in a relationship, because you are neither of them (and they certainly don't confide in you). But to answer your question, there is no strong evidence availible. In-game dialog hints that, if not romantically involved, they will be soon, but, again, nothing is explicitly known.

As to why Starcraft 2's Jim / Sarah relationship seems so different, there are two main reasons. The first is that we are, for the first time, seeing things from Jim's perspective (which we didn't get in the first game), and second, it's been 10 years since starcraft 1, and Blizz has a new crop of writers handling the story.