I have both slackware and windows XP set up as dualboot.

Is there any way to transfer files between them, if so, how?

You'll need a third partition of type vfat.
Alternatively, you could use removable usb media.
Another alternative is, depending on your filesystem types, the ext3 driver for Windows, but I don't know if it is read-only or not.
Another alternative is the ntfs-3g, or build and install the newer ntfsprogs, but I'm not sure I would trust either of them with writing to the XP system partition.

There are lots of ways, depending on if you are transfering too or from windows. If you are trying to pull files from windows, you can mount the windows drive if you use the ntfs kernel module. It is probably present in your kernel source, but not active, but if not you can get it here (http://www.linux-ntfs.org/doku.php). You'll have to compile your kernel by hand after any updates, and last I knew it was still not safe to write files to the ntfs system.

There are a couple options for windows, which are read/write, so it might be your best option.
http://www.fs-driver.org/
http://www.diskinternals.com/linux-reader/

I only cared about reading from windows, so I used the ntfs kernel.

Since then I have stopped dual booting my machines and have moved to using a combination of cygwin and virtual machines, depending on what I am doing.

Normally, I will allocate a VFAT partition, e.g. /data, so that I can access this partition both from Linux and Windows happily. So I think it is important to plan how you gonna use your machine before you actually install anything on it.

Install this package:

http://www.linuxpackages.net/pkg_details.php?id=11248

then add this line to your fstab file

/dev/sda1(yourNTFSpartition) /yourmountpoint ntfs-3g defaults 0 0

and you shoud have no problems reading and writing to your NTFS/XP partition.

There are several solutions, and you'll have to find out which one is best for you.

1) Are you planning on using Windows just until you are familiar with Linux?
2) Will you need Windows for the foreseeable future because of some legacy application?

If 1) is your situation, you can use ntfs-3g on Linux to access your files on Windows. I have used it in several situations to read (and write) data on ntfs partitions without problems. You can also use Ext2IFS_1_10c from Windows to access your ext2/ext3 partitions from Linux (without security...)
In many cases I have used these solutions until the user was 100% familiar with Linux and then simply removed the Windows partition and used the available space for other causes.

If 2) is your case, you should consider a vfat partition. It's still the safest option. I have installed systems with Windows XP / Slackware and a vfat partition for shared data. For instance, you can setup Thunderbird on both OSs using the same 'profile' on a vfat partition so that you can read e-mail from both systems. If you want to share your favorites between XP and Linux, consider FoxMarks and FireFox on both systems, that's how I usually install dual-boot setups.

Hope I have been of any help!

Edit: typos...

Thank you for all the help, guys.