Yeah, as long as you de-authorise the software on your old PC you can install on a new OS normally.
Windows software is windows software, especially newer stuff. If it works on XP it'll work on W7. Actually, thats a bit of a generalisation... some older stuff might not work, but Elements 8 and Lightroom 2 definately work on both. I'm using both on the 64bit version of W7 too. Don't know anything about plugins so can't comment on that.
Read this section on the Adobe site, should explain it all.
Canon EOS 50D/EF-S 18-55mm IS/EF-S 55250mm IS/EF 50mm F1.8 II /Lowepro AW Slingshot 200/ Digital Ixus 860 IS
PC>Q6700/MSI P45 Zilent/6GB 800mhz DDR2/2x 4870 1GB/Samsung Spinpoint F1 500GB/Maxtor 200GB+80GB