It depends on how the 45s were recorded. If it were on one of those USB turntables that are popular now, nothing can be done. They are pretty f#cking crappy and have horrid cartridges. Older 45s sound better when lifted with a good cart and a good needle. Especially if they are older 60s 45s recorded in mono. There is a right way and a wrong way, and if the signal was put on the CD via a newer turntable, you have to go find the original 45 and do it over. I had a guy send me a CD of some old lps that he recorded on a USB Numark, and they went straight into the trash after I listened for about one minute.
These were recorded on good equipment, just had some overzealous EQ applied. Joe is going to listen to a couple of tracks and see if anything can be done.
I want to point out something about those USB turntables: the main problem isn't the cartridge or the stylus, it's the USB. An simple USB connection is apparently a big selling point, but a USB connection bypasses your computer's sound card. There's a circuit in line with the USB that acts as a sound card. Problem is, it's typically nowhere near as good as your sound card.
Solution: most of these turntables also have RCA outs so you can connect to your receiever. Go to Radio Shack and buy a Y cable with stereo RCA plugs on one end and a mini-jack on the other end. Use this instead to record through your sound card. Unless you have a really good USB turntable and a really crappy sound card, there should be a noticeable improvement.
The cartridges aren't audiophile at the $100-$200 price point for the whole turntable, of course. But they're decent enough if you record through your sound card.