Guinea pigs, by and large, don't need regular baths. That's not to say that they couldn't benefit from one every once in a while, but it's not necessary.
There are, of course, exceptions. The most common exceptions include:
guinea pigs who are being treated for mites, lice, or skin irritations
long-haired guinea pigs whose fur is not trimmed up above floor level (thus being left to drag through cage bedding)
guinea pigs who have gotten dirty as a result of diarrhea or excessive urination
long-haired guinea pigs that have been poorly groomed and brushed
guinea pigs who have been living in/rescued from filthy living conditions
Some guinea pigs enjoy a good, warm bath. Most protest over some part of the experience, if not the entire affair. If there's a part that they hate, it's being dried by a hair dryer. You can understand why -- most handheld blow dryers are noisy and hot.
Enter the Vidal Sassoon Answers blow dryer for fine hair. Created for humans with baby-fine, limp hair that's prone to breakage at the hands of brushes, dryers, and curling irons, this dryer is also an excellent product for anyone who has to groom small pets.
Its light weight means it's easy to hold with one hand while you're using the other hand to keep a wet and very peeved guinea pig from wandering off. It's significantly quieter than the typical hair dryer, which means it won't hurt your piggy's sensitive hearing or drown out the reassurances you coo to help steady rattled nerves. It circulates effective drying air without making your piggy feel like he's standing in front of an industrial turbo fan. And its heat settings are not nearly as hot or harsh as those found on the average for-all-hair-types dryers. Lastly, there's the "ion output" -- we're still not entirely sure what that means but do know it has something to do with the fact that your piggy's coat is soft and shiny when it's dry.
This product can be found online and at stores like Walmart for about $20.