Old modules can be very playable, but I think htey work best when you take liberties to make them moreinteresting in 3e
Add prestige classes where necessary. OD&D didn't have them at all but they can be fun for flavor. Always watch CRs carefully, adjust monster strength to fit your PCs.
Remember, that while many monsters are tougher in 3e than in older editions, also consider that older modules were designed for larger adventure parties.
One particular thing about Basic and Expert modules, and to a lesser extent companion and Master modules:The list of monsters available in the core rules was very short, even comared with AD&D. AD&D had a separate hardback full of monsters, pluss supplement hardbacks. OD&D's monsters were all in very short chapters in the boxed sets. IMO this led to choosing some monsters that might seem inappropriate for D&Ders used to a larger list of monsters. Some modules compensate for this by having interesting new monsters (X4, X5, B3 stand out for example)
I'm currently running X4-5 (see my sig). I've done a lot of things to make it more interesting to 3e players. spoilers follow: I'm making several prestige classes to represent elite servants of the master. I'm using the Zhenturiam Skymage to represent the mage flying the wyvern in X4, and have make such mages a regular occurance. I'm using ogre war hulks (from the minis handbook) as an elite force of ogre warriers of the master's army. I'm going to create one/two PrCs to represent the secret agent Diviners of the master. I've swapped the monsters in numerous places where I feel appropriate. I"ve made the monster make-up of the Master's army less 'random' (it seemed to be a potluck of every evil monster in the OD&D rules); I made it mostly humans, ogres, some giants, orcs, and a few fantasic beasts, plus some exotic mounts like wyverns and axebeaks.
I have yet to convert the Great Pass section of X5, but I'm definitely going to change the monster selection there. Again, some monsters just seem to be there for no partiuclar reason. The medusas for example. I don't want to runn a Greek-myth themed campaign. So I'll probably drop them.