|The closest you are going to get to ReadyBoost in XP is eBoostr:|
This is commercial software and not free. I don't know of a free alternative.
I am not a fan of performance improvement software. They make great claims but rarely deliver on them.
ReadyBoost and Eboostr essentially use a flash drive as a disk cache. This works because they have very good seek times which is usually the most important factor in disk performance. How much this will improve performance depends on many factors. I wouldn't expect much. More RAM is usually a better alternative. If you have 2 GB or more RAM this product is likely to do little.
Many people have tried a flashdrive as a pagefile. But this will only be a significant benefit if pagefile performance is a bottleneck to performance. It usually isn't.
If you want better performance then add RAM. If this is not an option you need a faster computer. A flashdrive is likely to do little.